I'm surprised this topic wasn't a big one. I'm gonna twist it a bit and just list everything that satisfies me on Halo 3 MP. There's LOTS of satisfying things on Halo 3 I could go on all night with, but I'll list some of those ones that give me a real kick.
1) Winning a BR fight against experienced opponents
2) Getting a bulltrue
3) Ending that guys spree in the Warthog who was winning his teams game alone
4) Sticking somebody when your shield is down and then surviving
5) Headshots with the sniper
6) Getting 4-shots with the BR
7) Coming 1st in a Lone Wolves game
8. Becoming a new rank you really worked hard for
9) Lasering Warthogs, Banshees and Choppers
10) Multi-kills higher than a Double Kill (for me, personally)
11) Sniping the guy with the other sniper on The Pit, Valhalla or Last Resort
12) Getting to the camo, overshield or winning the battle for the rockets before anyone else gets them!
13) Making your opponent fall off the map
14) Shooting the generators behind an opponent and making them explode when you know outgunning them is impossible!
15) Scoring the final point in CTF
16) Achieving a very high spree!
17) Picking up the overshield and being attacked when it's charging!

18) Outwitting or surprising an opponent by using tactical jumps
19) Being MVP
20) Winning that sword battle that went on for half an hour with B, B, B, B, B