I do wish I had the monies to be able to re-play halo 2(without all the rampant cheating). I am pretty sure I had the most fun playing that one, but it was ~10 years ago, rose tinted much?
Auto-aim sniper swipe kills! hmmmm
I still feel halo 2 had the best map variety, small, big, symetrical, asym, just really good layouts.
Halo 2 was also the last "tongue and cheek" slightly silly, not serious campaign. So that holds up.
Also very few invisible walls, I loved just walking around outside the map, both campaign and multi-player.