In my (limited) experience, results of converting M42 lenses to Fuji have resulted in "very average" results vs native Fuji glass. So if you're doing this for financial reasons, you can get some great bargains out there. But if you're doing it for quality reasons, (again IMO) it's not worth it.
"Best" probably needs mode definition. Best as in volume of lenses? (M42 probably wins this in a race with Nikon) or best in terms of quality? I love my 28/50 FD glass but it's still no comparison to normal Fuji lenses. The only adapter I have is Leica-M -> Fuji to use M mount lenses on Fuji X. Everything I've adapted has compared favourably to Fuji native stuff (50mm Summicron & Zeiss, 40mm Nokton, 28mm Summicron and 35mm Ultron).
At 50mm for example, I've tried a CZJ Pancolar 50mm f/1.8 (M42). I paid sub £100 for the zebra version. It was definitely ok. The 50mm Canon FD f/1.4 is a lovely lens. It's around £100, and again
very ok. Canon EF wise, I've tried the "old faithful" 50mm f/1.8, f/1.4 and Sigma f/1.4 ART. I wouldn't bother adapting any of these mainly because at this price you may as well get native Fuji. The Fuji 50mm f/2 sells for about £300
second hand and is gorgeous, light, weather sealed and AF. Finally, the Leica Summicron 50mm f/2 is stupidly expensive, and totally not worth the price hike over native Fuji glass unless you need manual focus, a smaller footprint, or want-Leica. If I didn't have the money for a Fuji native 50, I'd get the Canon FD 50mm f/1.4. If I had £000s I'd get the Fuji 56. And I have no experience of LTM, Minolta/Sony, Nikon or any other thread mounts...
If you already have some existing lenses, then an adapter is a very cheap way to get access to a range of focal lengths. If I was starting out, I'd probably get an FD adapter because I have been really surprised by the quality of those lenses - especially at 28 & 50 which are my two primary focal lengths.