For me is the best wheel size, the most balance and the most fun. I´m not too tall so I don´t like 29", I only own a 27.5" and 5 26" the newest is a 26". My last built is a declaration: I don´t care about 27.5 or 29".
Recently I have built some custom wheels (CK hubs+ Alpine rims+ CX ray) because no one have a decent wheel in 26" the best for XC you can find is a Cross ride. Why should someone expend lot of money in some good wheels if he think 26" is death.
But not all is XC, for free ride spank still sells 26" wheels and I think hope do the same. And for street trial 24" or 26".
And except in mtb clubs, people still rides 26". There are lot of 26" bicycles and even Decatlhon sell one model or two in 26". Magazines and shops what you to buy the last crap they have invent, it is your choice. But after lot of years in MTB you will feel tired about the last, the best what ever. It is always the same: your bicycle is a crap because it is not the last bicycle ...