"I never do this" -- said while you are in the act of doing something completley unlike you. Perhaps you're the "good girl" and you're sneaking into the community pool for a midnight swim. Or maybe you're ordering a really decadent dessert.
I never did this -- I find this an unlikely sentence.
I've never done this before -- easily understood.
I never did that -- a denial
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.