I'm with RejZor...I've been playing the heck out of the Orcs Must Die! series. Although....at this moment in time, I'd probably lean towards liking the first one better, but I'm only a couple of hours into the second one so my opinion may change shortly...
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I'm also doing a first run through Call of Juarez: Gunslinger and liking it quite a bit. Essentially, I'm playing the CofJ and Orcs Must Die! casually....
What I'm really into at the moment is Fallout: New Vegas and I don't see this changing any time soon. I'm hooked...and I'll probably be playing it for months.
Absolutely love the game.
I was actually going to start the STALKER series or Mass Effect series, both of them having been sitting and collecting dust in my Steam inventory for a long time...but I wanted to try this game out after I upgraded my laptop to an A10 processor...and as I was checking the FPS under different settings...I started liking the game more and more...until I sat there for 8 hours straight. Something I haven't done in years.
IF I get bored with it down the road...I'm going to start up the STALKER series, but I'm currently trying to check out the mods for this series and I could use a little help(IE: links, which mods are best, which ones I shouldn't bother with etc...).
Best,
Liquid Cool