B-K: Freezeezy Peak. It's the Xmas level! It's the perfect stage you can point at if you want to shame people for preferring Mario 64. You just say "Mario's got two snow levels, one with the shed and the other with the snowman. Banjo's got one snow level with Xmas Trees, Xmas lights a Snowman, evil Snowmen, a giant scarf, a Xenophobic Walrus, a Deadbeat Dad Whose Not Got His Kids Presents (literally the plot of Jingle All The Way), giant Xmas presents, sledding, an igloo, happy little cottages and talking icicles who warn you about how cold the water is. Banjo levels are STUFFED with atmosphere and detail. They're rich and full of life! Freezeezy Peak is the easiest example of it.
B-T: Cloud Cuckooland. I like how it's a random mash up of random stuff in the sky. I also like how there's no backtracking needed for anything within the level, real nice that. Not that I have a problem with backtracking, but after a whole game of it Cuckooland always came as a nice relief.
Project Ukulele: Hmm. Well if you asked ME what worlds I wanted to see in a collectathon I'd say "How about a Target world" but I imagine PlayTonic wouldn't do that because they're in the UK and might not have Targets. It's a retail outlet that sells everything! Like a Walmart only moderately classier. You know what Walmart is, right? They're apparently called "ASDA" in the UK.