Suprisingly, I'm actually going to nominate Super Mario 64 for "best first level". I don't like the game as much as a lot of other games, but I have to admit that Bomb-omb Battle Field is one of the most well structured introductary levels I've ever seen. It's not hard and eases you into the game's mechanics, but it also has a lot of secrets and translates the grass theme to 3-D very well. It's the only level in the entire game that I find myself going back to a lot- and I don't really play the game that much at all.
Worst? That's a toughie, but I'm actually going to say Super Mario Brothers: The Lost Levels. Yes, I've never gotten far in the game at all, but when I look at video game maps online, the later levels seem a lot better than the first. Level 1-1 honestly just feels too cluttered and rushed, with no rhyme or reason to the design. This is again compared to the other levels in the game - Lost Levels seems to have some of the best and most devious level design in any mario game, and the first level just doesn't seem to accurately convey the depth and scale of the rest of the game.