Man, there were so many good levels in the ol' 64 version.
Personally, I really liked the original Metroid level. Brinstar is fairly small in comparison, and the only way you can die by falling on the original level is if you have a star... whereas in Brinstar, often the acid's too far down and you die anyway, which kind of defeats the purpose of the stage. So, if I had the choice, I'd swap out Brinstar for the original metroid level in a heart-beat, as far as Brawl's concerned.
Saffron City was also executed way better than Spear Pillar. They both have the same "random pokemon attack!" theme, but the fact that you can (a) see the pokemon in Spear Pillar (b) rarely do anything about some of their attacks, like stage-flipping and those projectiles... it just kinda ruins the point. In saffron, you'd only get pokemon attacking you if you were thrown/went into that shaft... and if you did that you kinda deserved it. Even without the pokemon, Saffron City had a really cool set-up.
And then there's the secret mushroom kingdom level - I'll never understand why they didn't carry that in future smash brothers games. It's the only level who's unique element was never emulated or replicated in future games (being the pipes), but it was ~so~ fun. Also, those pulleys where you sank/lifted up depending on weight? Those were hilarious! And now that I'm thinking of it, I'm remembering even more great stuff about that level... the piranhas that'd come up and blast you? The devastatingly powerful pow-blocks? Man! That was the best.
