Here's how I see it. I was occupied with Halo Combat evolved for a little while, but only with friends. I thought Halo 2 took a step in the right direction with custom games, but it was annoying to have to fit them to the maps and having to remake them every time you turned the game off. Halo 3 was by far my favorite. In every Halo game you care to name, excluding ODST, I thought the campaign was sub-par. I always just beat them first to get them out of the way, then focus on the multiplayer. That's why Combat Evolved and ODST were my least favorites. ODST's online was fun for a while but there was no variety so it quickly dies for me and my friends. Combat evolved was always fun, but again, no variety. 3 was my favorite because of forge and custom games. Again the campaign was boring from start to finish and matchmaking died down for me after a while, although the new maps and forge variants of those maps with updating gametypes has brought me back a few times. Now, however, custom games is the only thing me and my friends do. We haven't played for a while, partly because of Christmas and L4D2 (love it), but if we ever did I'm sure we'd go do some awesome infection gametypes and actually have a lot of fun. I've gotten more playtime out of Halo 3 than any other 360 game I own, largely thanks to the creativity of the fanbase. Of course some of them are huge jackasses but that's an inherent problem with the XBL community. Oh yeah, and as for Halo wars, that's actually my second favorite of all the games, simply because I'm a huge fan of RTSs, and this is the only RTS I can play with my friends, as it's the only console one I own (The bastards refuse to buy PC RTSs). The controls are fairly intuitive for a console RTS and there's a large enough unit/building/general extras choice to keep it interesting.
So there.