h3 for multiplayer, h2 for everything else (music, artsyle, atmosphere, story, and just the campaign in general). though I do prefer one aspect of h2 multiplayer over 3, being hitscan.
Honestly I have about equal good times on all of the games, but I think at its release halo 2 impressed me the most. Plus I always loved that train map in multiplayer, did a lot of stupid stuff on those rails.
Halo 3 was my favorite of the original trilogy mainly because the campaign missions were all so much fun with no super short or bad levels, the multiplayer was excellent, and it introduced forge into the franchise. I mean, Halo 2 had the best story and CE has the best atmosphere, but they just don't hold the same place in my heart as Halo 3 even though I love them all.