My thoughts.
Ubers - Reasonable, not the most balanced but it was never really meant to be as it started as a banlist. Barely played this though.
OU - Pure unadulterated garbage.
UU - Easily the best metagame at the moment, balanced. Any style of play can succeed and there's no truly tier-breaking Pokemon. Reminiscent of DPPt OU.
RU - Underrated, it's a very balanced tier. Admittedly TR and ghosts seem to be rampart but again it's still not bad. Needs a bigger playerbase imo.
NU - Great tier, very cool as you can try Pokemon you like but were never really viable in Standard play.
It bothers me how Smogon managed to make most tiers decent this gen but created possibly one of the worst Standard metagames I have ever seen.