Personally my favourite Halo Game besides the first one. Tons of fun in the campaign and the fighting in multiplayer makes it fun. Especially if you find friends and others to play against :3 Just like the music - DUN DUNNNNNNNN That piano is just... Magnificent, the way Bungie left Halo was in a way that 343 has yet to make...
Overall, if you want to start with a Halo game, I'd say - Play in order of release but remember - Halo Reach is one of the best, if not THE best Halo out there ;)
And with talk of the PC version coming soon... There's even more to be hyper about ;)