For me, probably some that allow my character to block while dual wielding and allows first person view while riding a horse. Those are just some things that if I saw were available as a mod and the mod was good, I'd download them and use them because they'd only add to my experience in Skyrim.
The biggies would be something that fixes bugs of course and one that fixes the lighting if Beth hasn't improved on that. That has been my biggest beef with the past two games. It's just weird walking into a tavern, in the middle of the night, during a thunderstorm, only to find that it's suddenly bright morning inside. I'd be lying if I said I wouldn't be disappointed had I found they not fixed this.
Maybe a barber kind of mod too, so I can change my player's hair style, unless there might be a code or something for that?