I've played a good chunk of all of them, but not finished any. I probably got closest to the end on DA:O, didn't get too far in The Witcher 2, and have dumped some hours into Skyrim.
I found DA:O to have good lore, and I normally don't get very deep into fantasy stories. I found myself reading a lot of the backstory and books lying around. The races and such are well detailed and everything made things worth caring about. The combat was a little simple, however I did play this one on the 360 which is quite a bit different so it may not be that comparable.
The Witcher 2 had much better combat, but because of that felt kind of difficult. Boss battles are quite difficult, and while it's good for boss battles to feel epic they felt a little TOO tough. Still, it's a very nice looking game and the combat system is the best of the bunch. It's a bit slow and methodical but the timing and such is what makes it good.
Skyrim is the largest game by leaps and bounds. There's much, much more content in this game. It's sprawling and completely wide open and I love it. Between them all I prefer the game Skyrim is. The combat isn't as fancy as The Witcher, and the lore and characters aren't as fleshed out as DA:O, but it's good enough in those departments and the open world gameplay makes up for it. It's also very good looking for the tech it uses. Skyrim also has a huge fan base already and they're pumping out mods like crazy which make the game a lot better in different ways. This is something the other games don't have as much of, if any.