Mine would have to be Morrowloot 4E Edition. Sure there are mods that add so much more, but the (re)addition of hand placed loot into TES is a brilliant thing in my books. I just love having to actually explore and search for stuff for once.
[spoiler]Oh, and I swear to god: If any of you say Falskarr...[/spoiler]
EDIT: This is for any game, not just Skyrim
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As soon as I get Halo CE we all have to play this