I use Modern.
I haven't played a street fighter game since 3 alpha, and have spent more time playing smash bros than anything else, making Classic a difficult change. I very much enjoy the neutral game more than memorizing combos, so the auto-combos are nice too.
I really wish I could choose the layout of the abilities, though. Some use the same button but with a different delay or hold, and I wish I could just do both.
I'm really enjoying it. I haven't played street fighter since 3 alpha, and was very casual then. I'm just barely into silver with one character, and iron with another. I have to say that, coming from Smash Bros, the netcode for SF6 is fantastic. I've only had two games that were unplayable or disconnected.