I agree that Halo 3 is a masterpiece! I love Halo 3...from the campaign to the multiplayer! Definintely my favourate and most played game...certainly up until 'Reach' was released!
I hate to admit it tho (as I feel like a traitor) tho Reach does have the upper hand! Both campaigns are equally awesome and replayable, tho Reach has a stronger, deeper and more cinematic story...and the multiplayer is superb with the different skill sets and game types! Invasion and Firefight (matchmaking :D) Bring it on!
I'll be playing Halo 3 for a long time to come, though Reach now wheres the crown! :)