The worst in the series.

There was a period of time when SMB3 didn't exist. SMB2 was the best, until displaced a few years later.

Out of Super Mario Bros. (ie, just the NES games), or out of all Mario games?

SMB3 is the best of the original games, although I prefer 2 to SMB1 or Lost Levels.

Out of the whole series, I'd say either Yoshi's Island or Super Mario 64, but I haven't played some of the more recent games, so maybe I should hold off on that.

I played Mario 2 plenty of times throughout my life, and never complained. I liked the game a lot.

SM2 wasn't even supposed to be a Mario game. It was an other project Nintendo was working on then they reskin some of the game and made it Super Mario Bros. 2

Going through the doors was fun. I'm a big fan of doors and going through them.

And the opening-hawkmouth? Come on! That was sweet!