Hands down, Halo 3 is a must have.
There's a great deal of replayability and they've added some cool things not featured before in the other Halo games.
There's the map editor Forge where you can create you're own variant of any of the mulitplayer maps.
There's theatre mode where you can view games you have played and take screenshots of the game you just played.
4 Player co-operative play where you can and 3 of your friends can play through the campaign missions of Halo 3.
And there's different armor variants where you can dress up your Spartan or Elite character model with the armor variants you unlock through playing the game.
To touche on multiplayer, it's a real different game than halo 2. Everything seems much more balanced. It really feels like Halo Combat Evolved (the first Halo game). And the game is even more nolstalgic with campaign.
Sadly though, campaign is the things that really puts this game down (other than the idiots over Xbox live). They didn't do anything new and it feels extremely dull. The 8th mission though is the worst and for various reasons. It's just extremely annoying. The last mission though, really brings you back to your early Halo days. It's a driving mission, well the second half is. The first half of the mission has a supposed boss fight which is more like fighting a blind cripple in a wheelchair. yeah, it's that pathetic.
Things is, you're probably not going to be playing the campaign as much as the multiplayer. The mulitplayer always seems to save the game from a dull and boring campaign.