I know many can find them frustrating, but experienced gamers and Halo veterans may come to enjoy Legendary Difficulty.
I found some Legendary Campaigns unrealistic and immersion-breaking, but thoroughly enjoyed Halo 3: ODST's Legendary Campaign. It was my favorite one (followed by Halo 3's), since enemies' shields weren't ridiculously powerful (or AS powerful) and I didn't have to be totally reliant on the Plasma Pistol EMP and Headshot combo.
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Reach was hardest, then came CE, then 2, then 3. My vote goes to Reach. I never played ODST on legendary