Hardest to Easiest.
Halo 2
Halo Reach
Halo 3
Halo CE
Most to Least Fun.
Halo CE: The most fine tuned and well balanced legendary. This is the best way to play, and feels like the canonical difficulty even if Heroic is technically. The enemies are powerful, but you're a supersoldier and moreso than any other Halo, you feel unstoppable. It's also the only legendary where any combination of weapons are viable, you don't need a plasma pistol and headshot capable weapon just to survive.
Halo Reach: Second only to 2 in brutal difficulty, elites are terrifying and they will obliterate you if you make one mistake. Reach just has one of the best campaigns ever, this translates well to legendary. Not counting Long Night against it as the unfairness was a MCC bug.
Halo 3: Outside some extreme spikes in difficulty, this also felt very well balanced. But at the same time, the rise tinted glasses have faded a little and Halo 3 just isn't as fun as it used to be.
Halo 2: It's not at all fun, but this is entirely due to the lack of fine tuning and balance. It's not just the jackal snipers, which the safety of checkpoints lets you die enough times to memorize their locations. But you feel like a wet napkin, chief is literally the second weakest thing in the game, that's not a joke. If Bungie had two more years, and the 360 hardware. Halo 2 in general would have been the greatest game of all time. But as it is, this legendary threatened to destroy me.