Twisted Metal 2 had the biggest impact and was the best at the time it released, absolutely no doubt.
If you're looking to play a Twisted Metal these days though, especially if you like local multi-player, then this year's Twisted Metal for PS3 is great. Seriously, it got a bad rep for the single-player campaign and buggy online play, but the actual gameplay design is really solid.
I also still enjoy playing Twisted Metal Black.