"Both of them but as above post said its better to talk in grammatical correct sentence so mostly i use i am doing well , how are you doing! and if other say i am good i can understand its American / Uk English difference.
Haha at first I thought you were asking a more philosophical question- like, "which is more important- to "do good" or to "do well"?" That'd be cool, in fact, I'm going to ask that