First, welcome to the worderful world of film.
Second, commiserations on getting here via Olympus... its a tall and slippery slope from the mountain of Gods, that leads into a peculiar perversions and ever more expensive things you suddenly find you 'need' when before you never even knew they existed!
Third, now you have found 'proper' cameras, don't forget, that SLR's aren't the be-all and end all of them, even in the 35mm format.
Digital has 'cheapened' the pursuit, and re-enforced the 'convention' that 'serious' photography demands an interchangeable lens system SLR... which... few ever change the lens on these days... as it's a zoom... so welcome also to the world of Primes.
Onto the question.... what's my 'favourite' focal length, or what's my favourite lens? And further, what is my 'most used' lens?
Well... my favourite lens is a 12mm fish; it's just 'fun' and gives a completely different perspective you don't get with rectiliniar lenses, and I'm greedy, and it packs in SO much field of view.... provided you dont mind straight lines bending a bit! NOT my 'most used' lens, however.
That would HAVE to be a 35mm Zioko... but NOT an OM prime, but the standard 'fixed' lens in my XA2 Compact... topic I was leading you too.....
Back in the days of mechanical cameras, they were not so easy to make or make cheaply; and to make them 'good' and affordable, they tended to lack the 'features' we now take for granted, even on cheap pocket compact digital, like mega-range zoom lenses. They came with a fixed, general purpose lens, and for a very long time, 35 was THE 'favourite' number, and the only lens length you got, on a fixed lens 'compact', which, probably WASN'T a 'cheap' non enthusiast camera. 35mm itself was the step into 'serious' photography, happy snappers used cartridge cameras. But keep it a secret, eh? There are hundreds upon hundreds of fantastic, and when new, incredibly expensive and sophisticated 35mm 'compact' cameras, you can now pick up for pennies!
My XA2, when I got it brand new in 1981, cost, I seem to recall £90. A lot of money then; in fact, it was only £10 less than the entry level OM10 SLR and 50, and twice the price of a Praktika, 'My first SLR Outfit' with aluminium flight case, three prime lenses, tripod, filters and gawd knows what else, cost in Dixons! Ie it was still a 'serious' camera. And still well regarded enough to command, oooh.... £20 on e-bay, in really good condition!
Small, compact, and one of the few 35mm Cameras you could truly slip in your pocket; and with the sophisticated OM based Aperture priority Automatic Exposure Metering; it was a LOT of camera in not a lot of space, which did deliver SLR rivaling photo-quality.
Yeah! Mirrorless? Micro-Four-Thirds? Small-Sensor System digitals and all that! HEY That old AX2 of mine is barely any bigger than my Daughters 'Compact' cannon digital! BUT puts pictures on a 'full-frame' sensor for 'professional' quality reproduction! (AND I dont have to worry that the battery will go flat in YEARS, let alone HOURS! of picture taking!)
Consequently, that little 35mm compact, traveled almost every where with me for twenty years, and has had more films put through it than any other camera I own. So I can say with some certainty, that its 35mm lens is my 'Most Used'!
And I have to say, as an all-round do-it all compromise lens length, it does take some beating; Its 'just' wide enough for landscapes; about right for groups and candids, and yeah, little lacking in reach... BUT you have to learn to zoom with your feet, get close by getting close, IF that's what you want.
On the SLR's, 28mm has been the widest I have ever grunted up the money for, apart from the fish; you just get into a situation where you are shifting the goalposts and making your frustration more and more expensive, chasing lenses to cram more and more in the frame, while forgetting you can zoom with your feet!
On the long side, hard to say what length I most use, or like the best. Depended on the job at hand really. The Olly's, as said, I started with that AX2 when I was still at primary school. I progressed onto hand me down OM10's, which were dirt cheap when I was at uni, so could abuse them mercilessly; important bits in the bag were a pair of much vaunted, Vivitar Series One, 'One-Touch' zooms. Wide end covered by 35-70, long end covered by the legendary 70-210 'Macro', which were carried over when I progressed later to an OM4. Primes were so seldom used on my OM's, I actually gave the two or three 50's that came with the OM10 bodies, away!
But that was as much because I had aquired a rather nice and rather rare, Sigma MK1, which came with an M42 fit, Carl Ziess 50! Primes were out of vogue, and M42 screw was the preserve of the bargain basement ameteur, starting out with a Zenith or second hand Praktika; consequently, I started building up a 'period - prime' camera kit in parallel to my OM's, around the Sigma; hunting through the 'bargain buckets' of traded in kit in the camera shop when I went in for film!
Oh! The nostalgia! FILM... wasn't just a medium! Was a social activity! You didn't just point a black box at stuff and press a button; you had to PLAN to take photo's, possibly for days or weeks in advance; starting in the camera shop, buying FILM, which meant talking to a bloke behind the counter, and discussing ASA's and grain, and f-stops! But I digress....
In the M42 kit, I dont know how many lenses I have bought over the years; quite often I'd end up with a 'Lucky dip'; box of cameras and lenses and whatever other parafanalia they had found in thier loft, that Camera shop wouldn't give them trade in for; or had, and they only wanted a fiver or a tenner to get it back out again! You'd go through and dig out more likely gems and then spend ages trying to clean them up, to discover the insides covered in mould, or threads stripped or irises snapped; one such disapointment I recall was a 35mm Ziess, that turned out to be a dodo... biggest 'loss' was a 24mm Pentacon, I think; wider than 28 was always uncommon, and I think I paid a whole fiver! for the box of junk it was hiding in, only to find the aperture was beyond redemption.
Ones that got kept? a 29mm Pentacon. A 'Cheap' lens, but remarkably good; seemed most people thought that the extra mm and coverage a 28 gave was worth sacrificing edge aberrations for; so in that 'kit' that's the widest, and probably most used lens in the bag. Ziess 50? probably the best lens, but, when you have others in the bag, wider or tighter, its just a bit too middling. 135 portrait, probably has seen more use; while the 300, I discovered in one of those lucky-dip boxes, has only ever really been used 'for a bit of fun'... took some fantastic (for me!) moon shots with it on an adaptor on the Digi-Nik! Far sharper than the shots I got with Digi-Nik 55-300, that's for sure!
So what do you REALLY want to know? Cos question is loaded, and leading towards... "So what should my next prime lens be'?
And the answer is.... depends what you want to take a picture of!
If you want a general purpose 'do it all' lens, then the 35mm prime, is a good one to have on the front of the camera. BUT, if you are going to carry around a more cumbersome 'interchangeable lens' SLR... and only have one lens to hand... why bother?
And for what a 35mm Zuiko is likely to set you back; as the MFT folk seem to be punting prices up as they can use them on Digital, and specifically want the shorter lengths to suit the small sensor... take the hint, look at high-end old compacts.
You can pick up an old XA2 or XA3 for £20-£25 in almost unused condition, from a dealer these days, and that is a full-frame 35mm camera you can slip in your pocket, as or more easily than you probably could a 35mm OM mount. Little more expensive, you have things like the Rolie or Minox 'folding' 35's that are just as pocketable.
Worth noting, that the XA's one short-coming was that it did give some descerneable 'edge abhorations' though to be honest, I have only really started to notice them, as I have been going through my negative collection, digitising my old photos, hence looking at the full captured frame, corner to corner, rather than the slightly cropped to suit print size snaps I got back from the chemist when I first took them. That abhoration was due to it having an 'equated' 35mm lens; the field of view was that of a 35mm, but the lens was actually closer than 35mm from the film plane.
Not often mentioned, but SLR's have a similar design flaw giving similar issues; to fit the mirror and pentaprism in the camera body, between lens and film, they have similarly 'equated' or 'retro-focus' lenses; the lens to film plane being longer than the stated focal length.
One of the big plusses of better 35mm compacts and range-finders; they can have 'true' focal length lenses, that don't need any correction to account for mounting the lens closer or further from the film; so can deliver better image quality for the money...
Which brings me to the Rollie and Minox 35mm 'folders' that were barely any bigger than the XA2, that had true 35mm lens that 'popped out' when you opened the cover. Fantastic bits of expensive precision engineering, when they were current; seem to recall my old man buying a Minox 35, and it was over £300... these days? You can get one in working order for £50 or so.
Which I mention, ONLY because if you are thinking 'thirty-five'... for the money of a Zuiko OM fit, offers an alternative avenue of photographic explaration to your voyage into film.... 35mm compacts.
Whats the addage? Best camera is the one you have with you? As said, that XA2 was constant companion for twenty years... LITTLE BIT battered now, so retired to discrace the display cabinet with its battle scars... but I have two others! While I still keep a Konica C35 in the car. Slightly older and a little bulkier than the XA or a Minox, but a cracking little tru-length 35 on the front; and it can live in the car, and I dont have to worry that battery will go flat on me from having been there five years since last used, if I see something with snapping. Actually one of my more used film cameras, JUST because it IS so often 'to hand'.
If you want a second lens to carry in your pocket; when the 50's on the font of the OM1; then, make it a 28, if you want frame filling wide. Its about the most common and economical length to get hold of without breakng the bank too much; while the extra angle of view is so much more discernably 'wide' over what you get with a 50 as to not be disappointing; where a 35 can, be 'wider' but frustratingly not 'quite' wide enough, and if you are trying to fill the frame with some spectacular expanse of scenary; not always practical to try 'zooming with your feet'.
Going longer? again, 70, is a little close to 50 to make it feel worth while; 90mm is not that common, hence expensive, 110 again, not so often found; 135, was the portrait photographers favourite for head and shoulder shots and flattering big noses.... not going to give you a lot of reach to fill the frame with small birds at any distance, but, it's a useful short tele-length, and far enough away from 50 to be worth the while. From there, 200 or 250 are very useful 'long' lenses, for a lot of situations; beyond that? you really need to have a good reason for one.
So, my recommendation, from 'organic' experience, and what has ended up staying in my M42 bag.... 28, 50, 135, and maybe something 200-250-ish if you have a use for it often enough. (But add a fish for fun!)
Meanwhile.. for all round general purpose, do it all with one lens.... dont get a 35mm lens... get another 35mm (compact) camera, with fixed 35mm lens!! WONT get away with buying 'extra' cameras very often; but this is good excuse for one!