Which is the best RPG out of these...

What setting are you looking for? Old fashion fantasy with elfs, drawfs, and magic? Or futuristic sci-fi? If you prefer one setting over the other, you can eliminate 2 of your choices by default.

While Skyrim is getting a lot of positive feedback, you commented that Oblivion didn't do much for you. Oblivion got a lot of the same positive feedback. To be honest, Skyrim doesn't play much differently so it doesn't sound like the best option for you.

Deus Ex is a lot of fun. If you like stealth in your RPG, you can't go wrong with this one. I found it a lot of fun planning how I was going to get from point A to B undetected.

The Witcher 2 is an amazing game. I haven't finished it yet, but the game is probably the best graphically out of all of your choices. It is a gorgeous game. I'm enjoying the mature tone the story is told with. Of the games you listed, the gameplay feels most like a true pc game (not that the others play all that badly). You can't go wrong with this one, and this is probably the one I'd recommend.

ME2 - haven't played it. But I have played ME1. So if you enjoyed that, I've heard ME2 is even better. If you play that and beat it, you'll be ready for ME3 which isn't too far away, and is supposed to close the trilogy.

My personal new favorite isn't even a pc game, and I almost exclusively play on the pc platform. Dark Souls on the consoles is an absolutely incredible RPG. Not sure if you have a console, but if you do, do not overlook this. It may be the most hardcore RPG I've ever played. And it also happens to be the most addicting RPG I've ever played. I've only got about 15 hours into it right now, but from what I've seen so far, this game should be mentioned amongst the GOTY candidates.

I didn't read your post until after I voted. Knowing you don't like Oblivion, I highly recommend playing Mass Effect 1 & 2. Awesome sci-fi action with a great storyline.

not deus ex if you want to get your teeth into it (too short, but still very very good).
witcher 2, is a lot harder than the witcher and is different in style.
Skyrim is very open from what i've seen so far.
Mass effect 2 is ok, but you get to level up after every mission, as the mission bonus is equal to the level-up value, there's no reward for how well you do the mission, and so it seemed poor compared to the original.
Best RPG - skyrim, as there's always something else to do, you can carry on doing stuff after the main plot has finished.
Followed by the witcher 2, I don't see mass effect as being an RPG, i'd see it is as being a mission based shooter, i won't be buying ME3 until its cheap on steam. Deus Ex is similar in my opinion. Fallout 3 would come just after skyrim on this list.

Oblivion didn't do it for me. I kinda got bored of the game. From a couple of reviews that I've read, Skyrim is pretty much similar to Oblivion. A few comments I've read basically stated that Skyrim felt more like an Oblivion expansion than a standalone game.

I liked Fallout 3 because of the open world and vast amount of community created mods for the game. Fallout: New Vegas was good too, but I didn't play it as many times as FO3 (on my 4th game currently). I only played FO:NV once and I'm not sure if I will bother playing the last two DLCs that were released for it.

I suppose I will try to pick up Oblivion again (start from my last save). I already bought it so might as well try to get my money's worth. If and when I finish it I might consider buying Skyrim. Maybe by then the GOTY version with all the DLCs will be out.

Mass Effect 2, specially if you still have your Mass Effect 1 save. Import it. All the decisions you made on the 1st game will be reflected on the 2nd game. Thats really the what I liked in the Mass Effect series. Now Im waiting for Mass Effect 3. =)

The other games that you listed are all good. But if you dont like Oblivion, I think you wouldnt like Skyrim. Witcher 2 is good as well. Its the game that satisfied my Mass Effect craving. Cause if you play RPGs with moral choices that affect the game, other RPG without that kind of gamepaly really dont satisfy anymore. Mass Effect really set a high standard in terms of gameplay choices and NPC interaction.

You should try Witcher 2 atleast, its a underrated game for no real reason. Its not as free game as Skyrim, but since you dont appear to like bethesdas games, it should be no problem the story is controlled.

I have to say that I liked Dragon Age Origins more than all of those. The closest runner up would be the Witcher 2.

I haven't really played many modern rpgs; I enjoyed fallout 3 but found oblivion boring;
with much skepticism I tried out skyrim to see what all the hoohaw was about -- I think it's amazing it has a nice balance of running around doing w/e and bashing in dragons faces with sick looking maces :X it has 2 major flaws though - i. bugs, bethesda and bugs zzz but atleast theres a decent dev console to work around most of them. - ii. as you come to be more experienced with the game some of the imbalances really show and break the entertainment value quite quickly.
aside from guildwars though (which doesn't count) it is the only rpg I've ever played through with more than just my first character. Play this game, it's got dragons in it.