This is a list of some of the best levels found in B3313. Even if you don't plan to beat this game, or even come close, you should definitely check these levels out.
Each level has a difficulty rating from 1 to 5 faceless Marios, with 5 being the hardest.
Description: Probably one of the most custom and interesting levels found in the game. The level features a whopping 11 stars and takes the appearance of a factory designed to create the infamous Motos enemy.
Directions: Get to Challenge Lobby, and enter the door with the 2 on it.
Wiki Link: Motos Factory
Description: A very pretty forest level with some moderate platforming difficulty, but nothing too bad. Contains 4 stars.
Directions: Get to Challenge Lobby and enter the door with the 3 on it.
Wiki Link: Tall, Tall Treetops
Description: A fascinating level modeled after the inside of an SGI Indy computer, which was one of the machines used to develop Super Mario 64. There are 3 stars here.
Directions: Get to Polygonal Chaos and immediately upon entering the area turn left and go up the red ramp. You'll find an odd object referred to as "The Machine". Jump on top of it to enter the level.
Wiki Link: SGI Indy
Description: A beautiful custom-made foresty water level with great music.
Directions: Get to Plexal Hallway and enter the first door on your left with a 3 on it. Then jump into the painting.
Wiki Link: Grim Green Forest
Description: An odd water level featuring "ruins" and a scuttlebug statue. There's 3 stars in the area, along with a 4th star inside the ruins accessed via a door.
Directions: Get to Nebula Lobby and head straight up the ramp opposite the pipe you enter from. Go through the double doors at the top, then in the large circular room enter the door with the 3 on it, and enter the painting.
Wiki Link: Wet-Dry Ruins | Inside
Description: A compact but very cool looking cave found within Tall, Tall Mountain (beta). The area features multicolored mushrooms, a waterfall, and a small hut with a door. The TTM level itself contains 5 stars, one of which is accessed via the cave. The cave itself contains 2 stars.
Directions: Get to Royal Lobby and turn left, then jump into the large Tall, Tall Mountain painting. Scale the entire mountain to the very top, and jump into the hole at the top of the mountain.
Wiki Link: Magic Mushroom Cave
Description: A pretty coastal area with a small hut. Looks like a nice place to settle down. The area is hidden in a hole within the level Creepy Cove, which itself has 2 stars.
Directions: From lobby 2, head through the double doors up top and keep going all the way straight back until you reach a small painting of bubbles. Jump in. This puts you in Creepy Cove. Reach the far end of the level and underwater there will be a hole in the wall partially obscured by a red grate, but not completely. Enter it.
Wiki Link: Jolly Roger Coastline
Description: A level that's eerily similar to other Wet-Dry World towns, but with a blood-red sky and a large temple.
Directions: Get to Royal Lobby then turn right and right again to enter the painting depicting an ancient city.
Wiki Link: Downtown City
Description: A pretty water level with a unique bubble-popping feature. Very easy.
Directions: Get to Royal Lobby then enter the lower wooden single door straight ahead on the left. Enter the painting.
Wiki Link: Salty, Salty Shore
Description: A succinct yet well-designed tropical beach level. Not too big, but very pretty and worth looking at.
Directions: From Crescent (see the "How To Use This Guide" section of the Locations page for a route), then enter the left-most door and enter the painting found behind it.
Wiki Link: Bountiful Beach
Most of the content in this guide was created based on information found on the official B3313 wiki. Thanks to everyone that contributed to the wiki!