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
Changing the skin and adding decal/effects to certain Forge Objects.
AKA: Wallas would be allowed to change into a Sandstone, or , or different kinds of metal. You could also add decal to the object, like a frosting cool effect to make it seem snowy and cold, or a sandy effect to make it seem sand-like.