As Halo 3 is my favorite game in the series, I feel obligated to make a comment in this thread.
Halo 3 has arguably the best matchmaking and balance of any Halo game. Yes, there were some issues in weapon balance, but most were balanced out through distance and map control. Both in the social and ranked playlists matches were fun and competitive. Halo 3 was fairly up close and personal, but had the finesse the ranged gameplay needed.
As for campaign, Halo 3 has very memorable missions. Who doesn't remember trekking through the jungle, assaulting New Mombasa, driving through the desert on the Ark, knocking down the towers around Truth, and escaping High Charity with Cortana? Yeah the cutscenes were nothing special, but game playwise it was never just a sludge through (okay Cortana totally was), and each area and each fight was unique.
So yeah, my name is Sikslik7, and I'm a Halo 3 addict.