Currently 2. But 3 has the best Gunplay.
I knew you would say 2. Everyone always does. Lol. I'll never get it. I think I've only managed to beat the first playthrough once, never the second. I just can't get into it no matter how many times I've tried. I think the worst part for me is the weapons. I really don't like how they function. Same with some of the enemies.
3 is pretty great. Done that one a couple times but then when I get to mayhem mode, it's sometimes hard to decide which level of difficulty I wanna do cause there's so many different kinds of debuffs/penalties that I can't tell how bad they really are. Still a really good game on 1st playthrough though.
1 is my absolute favorite. I've probably played 50+ characters through both playthroughs with some DLCs easily. I can never get tired of it. Awhile back I even made up my own little speed running category (just for myself) and have been finding some really good strats where I'm usually just getting faster and faster.
Pre sequel, eh. It's good, I like it better than 2 but only had the desire to play it a couple times. Might try it again in the near future.