1. Halo 1 (Takes the most effort and skill) 2. Halo 3 (Loved abusing bugs and never getting caught) 3. Halo: Reach (Enjoyed the leveling system more than anything) 4. Halo 2 (I didn't really like this game. Combat was wonky and some weapons were way too overpowered. The skill level dropped significantly from Halo 1) 5. Halo: ODST (I couldn't even pretend to like this game. A friend had to beg me to play it with him)
That's my list. I haven't played Halo 4 yet, but I imagine it would be in the top 3.