Halo 4 was was nearly impossible on legendary, and wasn't very enjoyable because the enemies were ******* bullet sponges.
Heroic was 100x more enjoyable for Halo 4.
-I beat Halo 3 three times on Legendary but always co-op. I couldn't imagine having do it by myself though
-Reach (except for the final level ) and ODST were frustrating but enjoyable on Leg.