I <3 Halo:Reach - amazingly fun and best campaign + amazing Soundtrack
I <3 Halo 3:ODST - best soundtrack + Firefight + Coastal Highway and Mombassa Streets, amazingly unique campaign.
I <3 Halo 3 - best multiplayer levels + Forge + best Custom Games + soundtrack
I <3 Halo 2 - amazing huge fun campaign + soundtrack + Fun multiplayer
I <3 Halo: CE - started it all
Things from Halo:Reach?
The user-interface for the main menu.
Rewinding + film clips of campaign
Forge physics
Daily and Weekly challenges
The contents in the Armory
The file sharing system
Firefight options
Matchmaking playlist
Invasion
Headhunter
Custom Game options
The easter eggs
Assassinations
Looks of the weapons and vehicles
Falcon
Fall damage
like 60% of the weapons
Forgeworld
Campaign not disconnecting if someone leaves
Jetpacks and sprint for custom game use
Those are the things I could state that I think everyone could agree with, but like with everything, they should always add improvements.
[Edited on 02.29.2012 10:55 AM PST]