Man, Steam has sometimes prices so awesome that it is hard not to buy games, even if you got no time for them :( . Well, I'm about to start a new RPG from the ones I bought earlier this year, but don't which one to pick first.

Here is the list! In order of interest: 

Dark Souls 

The Witcher: Enhanced Edition & The Witcher 2 

Dragon Age: Origins 

Deus Ex: Human Revolution 

Mass Effect 1 & 2 

Star Wars: KOTOR 1 & 2 

Be very glad if you guys can help me decide.

How do you wanna play them? from awesome to most awesome or the other way around?

The Witcher games, especially the second one, are easily the best on that list.

The first Dragon Age (Origins) is very good as well and Deus Ex: Human Revolution is terrific.

Dark Souls is hard as f***, Mass Effect 1 has massive sidequest and loading time issues, strange visuals and annoying voice acting and the sequel is pretty much a cover-based 3rd person shooter with watered down RPG elements and a run-of-the mill sci-fi plot (continued).

Never played KOTOR games.

Out of those, I'd say The Witcher 1 and 2, followed by Deus Ex and Dragon Age for a tie. One could argue that Deus Ex is just as action oriented as Mass Effect 2 but then again, it never pretended otherwise.

If you want a difficult game? Dark Souls

If you want something more like Baulder's Gate? KOTOR (my personal favorites!)

Others are all great just pick and choose.

Dark Souls

Never played Dark Souls.
I really love the original Witcher, and Witcher 2 is great too.
DA: Origins is nothing short of outstanding.
I thought Human Revolution was a complete mess, but most people disagree with me there.
Mass Effect is a great RPG, and ME2 is an unforgettable experience, though not really much of an RPG.
KOTOR is extremely dated, so approach with caution.

I would recommend Witcher first, since it has a sequel which is already out and actually a great RPG in its own right (unlike DA2 and ME2).

Check somewhere else before Steam, especially with The Witcher games, as places like GoG or Amazon have them without DRM.

They also deal in dollars instead of euro's, when Steam had their last Witcher offers, they had the same figure in dollars on GOG, that can save you a lot of money if you live outside the US as the currency difference is pretty big.

And, yes, GOG versions are always DRM free.

Witcher 2 first. You should play all of them though of course.

talking about the Witcher.. Witcher 2 is on sale at GOG.com $5.99 as part of their summer sale