(SPOILER)The one that came in to my mind instantly was Airforce- the one you fight in the sky. There's 4 parts to it- Airforce, Speck, Laser Gun and Missiblebay. I'm not even sure if it's possible to fight all 4 sections at the same time, 3 might the most.

There's probably a boss with 5 sections later on in the game, but I'm going for Airforce still.

Damn it Del Muder! I liked the easy questions lol.