I think my favourite thing about Halo 3 was the achievement hunting. I rememeber thinking that recon armour was the coolest armour ever so I tried my best to get the Vidmaster Achievements. Most of them were pretty easy. But the one that I disliked the most was the one in Halo 3: ODST where you have to finish the last lever with 3 other friends only with mongooses. The most fun was in Halo 3 where you do basically the same thing except in ghosts. My friends agree that that was the most fun one. I enjoyed the co-op of campaign in general. I also enjoyed the multiplayer though I didn't play it much. Overall it was an excellent game. Thank you Bungie.
PS: because I am not home and I'm posting via my phone, sometimes placing pictures into the post is disabled for some reason, but I'll try.