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
ForgeWorld
Better Forge Options
Firefight customization
Space battle
Armor customization
Ability to have thousands of forge maps and films, other then the Halo 3 max of 100
The ability for people to leave campaign games withith the whole game ending, like in Halo 3.
Daily and Weekly challenges
Credits (to an extent), to make everyone happy, they should make credits for armor, but still show a real rank.
better graphics
Better animations
Assassinations
Gametypes (Headhunter, action sack gametypes, so really every gametype from halo 3, but with about 15 more added).
Better Network
Better Veto system
Better user-interface for the main menu
easter eggs
I could lable more stuff from MY VIEW, but those would be the options that people would disagree with me. The things I listed above, you would have to be trolling if you said they were bad.