A lot of pokemon have seen marked improvements this generation. Here's my shortlist of non-Mega favourites:
Slowbro, because of all the newly risen threats he can deal with (Azumarill, Talonflame, Mega Scizor).
Mandibuzz, one of the best defoggers, phazers and walls in the meta right now (Knock Off buff, Defog buff, Overcoat Buff)
Clefable, getting mono-fairy typing. Great as a Status Absorber cleric (and Moonblast + LO hits hard)
Roserade, getting huge set versatility from the change in breeding mechanics, and one of the winners of the HP nerf
Rotom-Heat, counters a lot of threats (Talonflame, Scizor, Mamoswine) and absorbs 2 kinds of status
Chatot, getting access to two super-good STABs (it's not OU, maaaaaaybe UU, but Boomburst and Chatter are scary stuff)