Fun fact: 9/10 users with no linked gamertag are better than you at Halo. That doesn't necessarily mean that I am; but it does mean there's a pretty solid chance.
Personally, I found Halo 2 the hardest. 3, ODST and Reach were a walkover, I never played Wars on Legendary, and I remember CE being tricky but after playing Anniversary I'm not so sure.