It depends on the game, many games (especially previous ones) would say POWERED BY NVIDIA or whatnot. Usually specific series get special treatment, Hitman goes AMD but the Witcher goes Nvidia or whatnot. To my knowledge that generally means that A) they're using hairworks or something, and B) they likely got to work with that company for extra optimization.
Then I believe the drivers also optimize games without the developers involvement (But I would assume this isn't as effective as working directly with that company).
But generally certain games "work better" on AMD/Nvidia based on whoever they worked with/built it for, however the majority are "better" on nvidia for various reasons.