It's hard to narrow it down to just one, but I'll list my favourites from each game:
CE: Keyes. This level is chaotic but fun. You don't have any allies, so you get to destroy anything that moves. I like it more than Two Betrayals because it's easy enough to navigate, somewhat short, and the hardest enemies in the game make their debut. The atmosphere and the chaos tell you that has officially hit the fan in full force.
2: Gravemind. This may be one of the hardest levels in the whole series, but it's a cinematic masterpiece. Like Keyes, the toughest enemies in the game make their debut here, and you witness the Covenant fall apart firsthand. There's no exposition, no cutscenes (I'm looking at you, Halo 5), you see the chaos unfold around you as you play the level. When the Covenant is fighting each other, you can let them duke it out, pick a side and kill the winner, or mow everything down yourself. Many people hate it because the first battle is excruciatingly painful, but my opinion of it hasn't been tarnished by trying it on Legendary... yet.
3: The Ark. I can see why people love this level. You start out sniping enemies with a squad of ODSTs, and then you get to cruise through the desert on vehicles until the last quarter of the level. You can find more Fuel Rod Guns than you know what to do with, and have so many options in each encounter. Playing this level on Legendary is a blast, and it's even more satisfying when you're able to keep any ODSTs until the end.
ODST: Data Hive. It's an odd choice, but it's because of the story. Throughout the game, you learn what happened to your squad but don't really have a clear idea of what the mission was until you finish Kikowani Station. Then you hear Dare's SOS and fight off a squad of Brutes led by a Chieftain to descend into the Drone-riddled depths of New Mombasa. The eerie atmosphere and the culmination of your efforts have led to this and the effect is intoxicating.
Reach: New Alexandria. The level's mood and atmosphere send the message that humanity is losing and hope is lost. It's also pretty fun to fly the Falcon and shoot down hordes of Banshees and Phantoms with the chaingun, and the secrets scattered throughout the level are entertaining. If nothing else, I like this level because I can party with Grunts and Brutes in the nightclub.
4: Reclaimer. You can do some pretty cool skips and there are almost no Prometheans you have to fight. It's one of the least painful missions in the game.
5: The Covenant. This level is so much fun. You start with a Spartan Laser and- wait, you mean writing +2 on my Halo 3 disc doesn't count as Halo 5? Damn. Then I guess the winner is the Breaking, not because it has fun gameplay or a huge impact on the plot but simply because it has awesome music, pretty lights, and I got the achievement for beating it on Legendary without dying on my first try.