I really need to give this series a go at some point.

Maybe I need to replay Halo 3 one of these days.

I remember being very disappointed with it when I first played it back in 2009, but maybe I'll enjoy it more a second time around.

Halo 1. Mainly because it had such a mysterious atmosphere. Great AI and just enough different weapons and options without being too crowded.

That and it has by far the best story in any Halo game. When the flood enters it's very surprising because when you play the game you're thinking this is just going to be another bro shooter against a single alien threat. It kind of is, but it has that nice twist that adds to the whole experience.

The A.I Healing stuff didn't really bother me, probably because I never really needed it anyway.

Should also mention... The Multiplayer was okay. But it was also incomplete.

Only a couple maps and game modes doesn't fully flesh a game out, granted... They did roll out more maps, modes, forge and theatre... But that crap should have been there from the start with new original content coming later that we could really look forward to.
..By the time all that stuff was added to the game, myself and a large chunk of the player base had moved on anyway.

I find running through it via split screen co-op is the way to go.

Fair enough. I do hope though that people aren't voting based on the multiplayer of 4 and 5 when this thread is specifically about the campaigns alone.