- Dragon Age: Origins + DLCs + Awakening (Awakening being the expansion pack)
- Dragon Age 2 (yes I do suggest it anyway, but beware it's nothing like the first, additionally one of its DLCs is worth it in my opinion, namely Legacy)
- Mass Effect 1 (must play, in my opinion, better in many aspects than the sequel)
- Mass Effect 2 (superb on its own, better in some aspects than the original, plus some of its DLCs are worth it, namely "Shadow Broker" and "Overlord")
- Portal 2 (you don't have to play the first to appreciate the second, although the first is still good enough on its own, although very short, sadly Portal 2 is also very short)
- Half-Life 2: Episode One & Two (if you haven't played the Episodes that is)
- Fallout 3 + DLCs
- The Elder Scrolls V: Skyrim
- Crysis (its campaign has a decent story, quite generic sci-fi stuff but decent nonetheless at least in its presentation, there's a bit of mystery and good atmosphere in most maps, one of them specifically was considered by most players to be the worst map in recent FPS gaming, but I myself thought that on the other hand it was refreshing and very enjoyable)
- Crysis 2 (I haven't completed it yet myself, I haven't bought it yet either I'm playing it from time to time when I happen to visit my cousin at his place and we play on his 360, but from what I've played it's "alright", it's "ok"... it just seems more generic to me since it's happening in a modern day American city, New York, so it has a strong déjà-vue feeling to it, but it's decent anyway, good action scenes, I might buy it eventually myself on Steam)
- Warhammer 40,000: Dawn of War II + Chaos Rising (expansion pack)
- Warhammer 40,000: Dawn of War II - Retribution (can be bought alone without buying vanilla DoW II or Chaos Rising, additionally Retribution uses Steam whilst DoW II and Chaos Rising use Games for Windows Live)
- Darksiders (sleeper hit for me, took me some time to get used to the controls though)
- Transformers: War for Cybertron (surprised me, I liked the campaign, it's a bit on the cheesy side of things when it comes to dialog and scripted events but overall enjoyable)
- Star Wars: The Force Unleashed (yes I do believe it's a good game on its own, mostly due to the story-line and the good voice actor doing Vader's voice, but be aware that the PC version is most likely the very worst console port I've ever seen, only buy it if it's on sale for extremely cheap, I wouldn't pay more than $5 for it, additionally, there's a sequel to it but I've never played it myself)
- RAGE (id Software's latest game, it's good, it's nothing jaw-dropping, better than the average generic shooters we keep getting one after the other since the past few years, it does have its strengths, but in my opinion it ain't exactly on the "addictive" side of things, let's just say that id Software tried something new and perhaps they shouldn't have tried that hard)
I know I'm forgetting a bunch I could recommend but I'm not at home right now, I'd have to check on my own games list I have installed on Steam to name a few more. But anyway out of the ones I and also others recommend I'd say that the absolutely must-play ones definitely are Dragon Age: Origins (DLCs or not, expansion pack or not, at least play vanilla) and the original Mass Effect (do NOT play the sequel if you haven't played the original first, do yourself a favor and play the original first PLEASE).