The smaller ones which add new features or fix bugs, like the horse armour type stuff in Skyrim.
Or the huge ones that add new stories or massively change the game structure, like the Aether Minecraft mods or The Second Reality Project type SMW hacks?
Personally, I've always preferred the latter. Better to get a whole new game for free than just to add a few minor features.
I use all sorts of mods, so I don't prefer any particular style. I'm sure most people have used all sorts of mods as well. Usually with a game like Skyrim, I start with only minor mods, things that say, improve quality of life. Like increasing carry weight, or making all ingredients weightless. Then on subsequent playthroughs I'll add on more stuff.
For Minecraft, I've never tried any mod that totally changes everything, except for textures. I tend to pick and choose things that I like, like adding in more animals, adding a mini map, being able to build rope bridges (I like to make tree houses, so rope bridges look better)... But then too, I hate modding Minecraft because of the hassle, so I tend to make as few modifications as necessary. I can't wait for their API thing, if they ever actually get it working...
