Difficult question, but I'd probably have to concede with Jakob3245 and say that Sword Cancel is one of the best glitches across the Halo games - that or maybe any of the super bounces/exit map glitches from Halo 2 or 3.
Managing to exit the map boundaries of Delta Halo was probably one of the most sublime times that I've ever had with Halo, along with the invisible wall bypass of Cairo Station.
I was once was playing big team slayer on halo:reach and i got into a wraith a killed maybe 10 guys then out of nowhere my wraith's cannon came off but i still killed 5 guys with splattering but eventually was boarded and died.
I was in halo: reach campaign, when all of the sudden I saw Emile and Jun, weaponless, with their shoulders back, and they were strolling right in the middle of a battle. yes, they got shot. no, they didn't die. It was hilarious. An example of this possible glitch can be seen in Fails of the Weak volum 11