The only game easier is Halo 3. Once you get a system down for dropping the Knights, you'll have almost no troubles except for a few problem spots. But all in all, its not too terribly hard if you take your time.
Don't be hatin' on Carmine, hundreds of them die every day to keep your ass safe from pale steroid abusers. - Sic_GloryFades
The easiest of them all. Legendary in Halo 2 was a nightmare, I couldn't do it on Halo 1 at all (solo), and Reach and 3 were pretty much tied. For most of this game, I found myself forgetting from time to time that I was even playing Legendary. It really only got tough whenever vehicles were involved. Climb into a banshee, and suddenly every shot ever fired on the whole planet seems to home in on you.
I haven't played enough of legendary to say for sure, but a friend has assured me that it rests between Halo Reach and Halo 3.
So, for reference, the scale my friends and I use from easiest to hardest:
Halo 3 Halo 4 Halo Reach Halo: CE Halo 2
A lot of this list differs from person to person, however. The only thing that I would cite as an absolute fact on that list is Halo 2 being the hardest. I firmly believe that anyone who doesn't agree has either never beaten Halo 2 legendary, or used a guide on the internet to glitch through it.
EDIT: A different friend uses a similar list, but places Halo 4 as more difficult than Reach, so there you go.