Ok, I don't know the names too well but from reading others, this is the deal. I included the level numbers after in case I got the names wrong.
3 - 343 Guilty Spark (level 6)
When the flood first come is really good shit. Nice atmosphere and enough guns without having library style repetition.
2 - Keyes (level 9)
Just after you get to Keyes and all hell breaks loose is really good fun.
1 - Two Betrayals (level 8)
This level kicks all kinds of ass. Backtracking sure, but it's just so good. Me and a mate always play this level on co-op on legendary. The first bit (where you come out of the tower) and then there's one of them big tanks and a ton of other guys down on the ground is class. The two canyons are both great fun and theres lots of gunplay and banshees.
I loved the Halo SP, I hope that Halo 2 is equal to it. Better would be bliss.