You should go through the series backwards (no joke). Halo 3 is the shortest and the easiest. If you really enjoy this and move on to Halo 2, the environments will be a bit less lush and it will be slightly more monotonous and hard. If you liked that, then move on to Halo 1. I haven't played Halo 1 firsthand, but I saw most of the game by watching my dad play. I can't make a fair statement about the difficulty w/o playing it, but there are a lot of copy-pasted rooms and the majority of the game is inside (you will only know why this is bad after you've played halo 2). If you really liked halo 2, then you should move on to halo 1.