Whilst I enjoyed all levels, lemme put it this way (Since I always play on Legendary):
The Library- The infamous level with countless waves of Flood with Rocket Launchers and Shotguns and basically every weapon in the game turning every corner and pinning you down and overwhelming you relentlessly as you repeated cry, rip out you hair, and beat your controller against the wall for hours. So many memories.
Two Betrayals- Constantly getting your behind caught up in a crazy 3-way battle between Sentinels, Flood and Covenant while you are being endlessly bombarded with Wraith Blasts and Banshee Strafes. Endlessly make you plan your next moves as others have failed terribly as you keep dying for the entertainment of others.
Keyes- Running around in the dark whilst keeping your eyes peeled for an ambush from the Covenant and/or Flood as you make your way to the Covenant Battlecruiser in the dark, narrow canyons watching as your enemies fight each other. As you enter the ship, you see swarms of enemies in the hallways and have the best choice of tossing a grenade in there, yet other dropped from the now dead alien's grenades are primed after the explosion and fly towards your direction crazily, blowing your face off around the corner. Rage ensues as you keep making the same mistake over and over again.
Yeah, I'd say Keyes makes me mad the most out of any level.