In my opinion, Halo 2 is still the best Halo game.
Explorability, replayability, and a fantastic mutliplayer experience. Not just competitively, but in a social way too. Superbounces everywhere, out of maps and various other glitches were part of what made Halo 2 so amazing.
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Gameplay was slightly hit and miss, but the story, art and music are some of the best in the entire series. My first real Halo MP experience was the H3 beta, so I can't really comment on that part of Halo 2.