Agent 9's Lab.
It feels like it serves no purpose other than to introduce the character we only got three minigames with(and one of those minigames was an FPS instead, so his introduction level felt worthless). And it just isn't that fun- Agent 9 moves slowly and clunkily, and the level was large. Bentley's Outpost has the same problems, but the level is fun and Bentley moves a lot better!
The best thing about the level is the fact that getting the egg from the boat was so easy.
And that's not really a compliment. I do like the design of the level and the music, but I like that with all the levels.
I also have mixed feelings about Fireworks Factory. I feel like it was overrated, with a cool theme but average layout, and I really hate You're Doomed. I also dislike how they recycled Handel and Gretel into basically the same plot as they had in Scorch from GTG. But on the other hand, I absolutely love the challenge with the Superfly/Superflame where you need to shoot down the dragons- it may be my favourite challenge of them all in the game.