I liked Halo 2 the most. No achievements, ranking system was brutal and therefore rewarding, community was creative and dedicated. Halo 2 also had the best maps, really, it's amazing that I can say that I don't hate a single map from Halo 2. It was also the only time in my 10 years of XBL that I could open up my friends list and find like 60 buddies online and all playing Halo where every other guy had a full lobby customs game going that I could join.
Good times for certain....
Too bad Bungie lost their map making abilities after Halo 2 and 343 is already showing bad signs of map designs. Hopefully the MLG peeps that were hired on at 343 give some truthful feedback about how trash the maps are. I still believe Halo 4 maps are better than Reach's but that isn't really an accomplishment to be proud of.