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.
I didn't think halo 2 was that hard.... AFTER THE FIRST F***ING LEVEL!! All those elites to fight with bad weapons.... Pretty sure I had to skip that one
I don't always teabag, but when I do, I use... DOS TESTES
Halo 3 (you can run past so much stuff lol) Halo: ODST (this game practically showers you with precision weapons, PP, and needler, which is all you really need) Halo 4 (there's some areas that make you think, but not mind bendingly frustrating once you figure out something to trick the AI) Halo: Reach Halo: CE Halo 2 (ive never beat it "properly", used terrain glitches on a few of the levels)