For A3+, my top paper would be Museo Max. It comes in 13x19" rather than 329x483mm. I'd class both as A3+, but you might want to check.. I was blown away by the quality compared to my Canson prints and the fact that it's a lighter weight paper didn't bother me at all. My 2nd choice would be Canson Infinity Rag Photographique (310gsm) or the Duo if you want a lighter, cheaper paper. The 310gsm is a nice heavy matt paper.
If you're on a budget - the Permajet FB Delta Matt is nice. Just done a couple of A3 calendars with them but mainly because it was cheap and they were... calendars. It doesn't have the richness of the above 2 papers though.
The Epson Velvet Fine Art is very good - better than the Permajet, but not the Canson or Museo and quite pricey. It has a profile for photo black as well as matt, which was a surprise.
The Epson Hot/Cold Press Bright (bright = whiter than "Natural") papers are massively expensive and I wasted my test sheets trying to get rid of the terrible magenta caste. And that was with their own ICC profiles and an Epson printer! I'd steer well clear of them. The Cold Press papers are also very heavily textured (it feels very artificial to me)
I haven't liked any of the Hahnemuhle papers I've tried - mainly because of texture/feel rather than colour rendition/sharpness.
Caveat: All papers tested with the same Epson Stylus Pro 4800 with Epson inks, manufacturer ICC profiles and Photo Black ink rather than Matt Black (don't have any matt black ink and not prepared to keep swapping even if I did). The Museo paper was superb in spite of that!
Re: Bright Whiteness...
I prefer papers without artifical optical "brighteners" This image shows a comparison of the "whiteness" of the matt papers and shows that the Permajet paper & the Epson "Cold" papers look "whiter". That's because they have an optical brightener added to the paper to make it appear 'whiter than white' as Dulux would say. I'm not a fan of this and avoid it which is another caveat against my recommendations. (photo taken on iPhone in natural daylight with no special attention to anything so please don't use as anything more than a comparison)