I think I managed to beat the first game on Legendary, a lot of tries were required, I may be wrong though as it was rather difficult.
Halo 2 on Legendary is too broken, enemies have way too good accuracy and stick yourself out of cover can kill in seconds, I think I actually tried sneaking past enemies.
I am pretty sure I have managed to beat Halo 3 on Legendary, though some bits did require some creative thinking like taking advantage of team mates, like giving them good weapons, the needle pure Flood were SOBs.
I know I beat ODST on Heroic *checks achievments* I seem to not beaten it on Legendary yet, I know that the hub world can be a bit pain and the loss of energy shield can be a problem.
I have beaten Reach on Legendary, actually was not too hard, use the right abilities (hologram mostly), use the night vision to spot foes and go at it, actually felt like the easiest Legendary campain.
Have not tried Halo 4 yet, mostly been playing multiplayer, I have gotten a rather good grasp of how stealth is meant to be used, use crouch while active camo is on to not be seen, although it does not work against Prometheans. I have tried my hand at the campain that is meant for multiple players by myself, can be kind of difficult, mostly been having problems with the prometheans. The little ones seem best dealt with a head shot weapon like Carbine to the glowing mouth, knights require sustained fire, or some good misdirection with holograms seem effective to get an assassination. I still think Jackles are best dealt with by a rapid fire weapon, shoot there opening then continue on their body, Elites still require some creativity good news is that taking them out still scatters Grunts for a bit, but the Hunters seem as tough as ever.
Now I have not tried on Legendary yet, still doing a single player campain on Normal, but I see no reason to rush. Still don't have much idea yet on the pros/cons of Assault rifle, Storm Rifle and Promethean version who's name I forget right now. Also not sure about the exact effect of the Promethean Grende yet.
_________________
Through dream I travel, at lantern's call
To consume the flames of a kingdom's fall