I'm not including World 6 because I don't want a million "that was barely a world, most of it sucked, only one level, etc."
If you didn't really like any level at all... well then I guess you don't need to be in this thread. The minimum requirement (apart from beating the first five worlds) is that you at least tolerated the levels.
I know most of you didn't like the game, but we can have one topic where we can talk about the parts we did enjoy, can't we? I don't think that's such a tall order.
World 1: I liked Goomba Fortress. It felt different from most of the game's levels back when I played it, as well as when I look back. It felt like it came straight out of one of the first two games. And the music reminds me a bit of Team Fortress 2.
World 2: I favored 2-1. I felt like the transition from grasslands to desert was handled well. I didn't care for many of the indoor segments later on in World 2, and this level didn't have as much of that, so I enjoyed it for that. I also liked navigating the big pyramid-type structure at the end of the level. And the Mariachi Shy Guys were interesting, if nothing else.
Runner-up level would be the oasis. Short and sweet, as well as relaxing (when damp).
World 3: Snifit or Whifit/Stump Glades. But I've gotten the impression that it's one of the more favorite levels, so I'll talk about my runner-up.
Whitecap Beach. I liked finally reaching the source of Wiggler and the forest's problems, the music was great buildup for the boss, and of course the boss itself was enjoyable.
World 4: Enigmansion. Again, I'm told this is another one of the few levels most enjoyed.
So my runner-up favorite is Bowser's Snow Fort. I enjoyed the Mario 64 throwback. I also thought waiting in line was funny, just seeing Mario stick out like a sore thumb next to all the Shy Guys. And I really liked the boss, it took me by surprise on the whole.
World 5: Rumble Volcano. Most of World 5 left a bad taste in my mouth due to an incident, but I knew who the boss was and that kept me going, and I wasn't disappointed. Petey's one of my favorite Mario bosses, so the chance to face him made putting up with the jungles before him worth it. And the actual volcano level was fun too. Not too long, not too complicated, and Kersti was largely absent.
1: Water's Edge Way - I came back here to get the Faucet thing so much that the place sorta grew on me.
2: Drybake Desert - It was a nice intro to the desert stage with a puzzle and an introduction to one of the few unique enemies in the game, the Mariachi Guys. I tried talking to them at first. They seemed nice until the fight started.
3: Stump Glade or The Bafflewood - Problem with the forest world is that it was all very much the same. Anything that was different was praised. The two mentioned stages got a few laughs from me.
4: I liked all of the snow world levels. I tend to like ice worlds. The boss stage was fun.