I might have said this before: This is engineering. There is no ‘best’. All there is is the least bad balance of compromises for the specific application. And as we’re talking about mountain biking, which is an incredibly diverse sport compared to road biking, then there’s no reason for numerous different wheel sizes to not co-exist, just as forks and frames come in a massive variety of geometries, strengths, stiffness, spring medium, sizes and so on…
I ride a 26er MTB. Quite a lot my riding buddies ride 29ers, though far more are on 26. Most of those I regularly ride with don’t change their bikes very often at all. I like the idea of 29er as a second bike for less agro riding (my agro riding being more about corners, jumps and drops than high speed rough descents – I’d totally get something like a Quarterhorse if I lived somewhere rocky) but I’ve been disappointed to notice the lack of mud clearance on them. And I like how short chainstays feel so I’m not sure how that can be solved!
650b may prove popular, it may not. In my opinion it’s close enough to 26 that it really doesn’t matter if one kills off the other or if both take a fair share of the market – by the time that happens I’m sure my current MTB will have been retired to an easier life. Much as I like using my LBS, I don’t buy tyres there and that’s the main scenario where too many wheel sizes is a problem.