Incoming wall of text.
Played since the release of Halo:CE.
On the campaigns: With the exception of CE and Halo 2, which were fantastic, I found most of the campaigns to be decidedly mediocre, formulaic, and forgettable.
On multiplayer: There will probably never be a multiplayer experience better than Halo CE PC. No game has ever taken more technical skill, no community has ever been more full of people who just love the living crap out of the game until the bitter end. I still play it, even now when it peaks at 200-ish people online worldwide. I outshoot hardened, experienced players like they're standing still; ten years of play will get you to that point. It's kind of an interesting experience. It makes me feel like I speak a dead language perfectly, or practice a forgotten black art that few others know.
I skipped Halo 2's multiplayer because I was still playing Halo CE way too much.
Halo 3's multiplayer was definitely a good time. The gameplay was a step down in intensity from CE, definitely a more chill style. I basically bought it because my friends wanted to play it, and in that respect it did not disappoint: Perfect game to just sit back in the chair, talk with friends, and mess around. MLG was an entertaining scene, but my memories are definitely more about whooping ass with buddies in BTB, social slayer, rumble pit, doubles, custom games, etc. I don't necessarily think this is the best game in the series, but it's definitely given me more fond memories than any other 360 title.
Playing Reach compared to playing Halo 3 is like eating burgers with your buddies compared to eating alone at a fancy seafood place. I don't think Reach is a terrible game (although the gameplay does leave something to be desired), but it emits no feeling of kinship or friendliness like past titles have. I don't have too many memories of awesome things that happened on Reach. It's a sterile experience. The game, in my eyes, lacks life, plain and simple. It does not FEEL like a Halo game.
[Edited on 12.24.2011 11:29 PM PST]