Halo 3 was the first game I played over Xbox LIVE. I did play Halo 2 as well at times, and I found it very fun, but I have a lot of good memories playing Halo 3.
This is obviously opinion, but for me, Halo 3 was the best. I love that Battle Rifle, the rank system, the maps, the feel of the game. Bungie did a fantastic job on this game from 2004-2007, and it seems that with Halo Reach, there wasn't much to "improve" on but they wanted something different. They felt that they (Bungie) needed to change things, but they didn't.