I think the fifth world of Super Mario Bros 3 is the cloud world, not the sixth one. I think it goes: Normal, Desert, Sea, Giant, Cloud, Ice, Pipe, Bowser's Keep. Not that it matters, I knew what you were talking about when you said "cloud".
SMB: 8-4(The princess ISN'T in another castle!)
4-2(Love the warps and plus it's already an underground level which has the cool music.)
SMB2: 7-2(I know the way through Wart's Castle, and almost all the music is that cool Underground music. ALL the underground songs rock!)
SMB3: 1-3(Warp whistle and so classic a level.)
Any Fortress(Great music, I can't deny it.)
SMW: Also Awesome.
SM64: Jolly Roger Bay(Also another personal song favorite, and the mist for the first star gives it a nice look.)
Wet-Dry World(Great Song, great strange look to it, great background if you look past the world's edge.)
Tall, Tall Mountain(Great slide, MUSHROOMS, very Mario-esque.)
SMS: Noki Bay(The songs and waterfall world and wall kicking redeem this world from boring. Very ocean-y)
Painta Village(So fun, okay song, big trees, MUSHROOMS, Night sky, everything!)