Halo 3 is probably the best, but my favourite is Reach. Spent the most time playing it, had the most fun on it, especially with friends and the custom games were amazing. Can't beat that customisation either
I love CE. The magnum is the great equalizer, giving a player a chance to compete even if he is being spawn camped by a team with power weapons and power ups. The game-play is fast paced and exciting and the levels work really well.
Although each one in the original trilogy is the answer because it only improved from it's predecessor (CE created it, 2 took it on xbox live and reinvented online gameplay, and 3 added forge, fileshare and films) I have a personal feeling for 2.
Halo 3 has the everything from casual to competitive with great weapon balancing and maps. Since there's no enhanced movement there's many different sizes of maps complemented by vehicles. You can't simply sprint away so each encounter matters on your skill alone (with some help of power weapons of course). Halo 3 has everything and no Halo game since has been able to replicate it's greatness. I'd go as far as to say that Halo 3 has the best multiplayer of any FPS game ever.