Vampire: The Masquerade - Bloodlines, patched to official 1.2.
If you read the reviews you'll see tales of a potentially great game, let down by a lack of polish and some infuriating bugs. The reviews were of the pre-patched version. Most of the worse bugs were cleared up in patch 1.2. If that version had been available for the reviews I'm pretty sure it would have been picking up 9s and 10s all over the place.
In short, it's a stunning RPG. The storyline is rather linear ,in a similar way to KoTOR, but you have a reasonable degree of freedom to take things at your own pace, in you own style and in the order you choose. Depending on your character build you could find yourself handling most situations very differently so it has loads of replayability. In structure and style it's the closest thing I've seen to the original Deus Ex (much closer than DX:IW).
It has the best script and voice acting I've ever seen in a game. Oblivion seems flat and lifeless by comparison. Coupled with the wonderful facial animation system of the source engine it seems like you're dealing with accomplished character actors (or maybe just characters!), rather than NPCs.
The storyline's pretty good, one of the best I've seen in a game. It has its cliches but, like many films, it's carried easily by the performances. If you like games to be scary, it has by far the most fear-inducing level I've ever seen.
It won't be over quickly either. If you do most of the sidequests it should take you a good 40-50 hours to finish, maybe longer. I'm on my third play through now so it's certainly value for money (I got it in the 'January' sales for a tenner on Christmas Eve 2004, only a month or two after its release).
Speaking of further play throughs, I'd highly recommend the unofficial patches. They started off with bug-fixing but the later versions have restored quite a lot of content that was left out late in developent. I'm playing with unofficial patch 2.4 now (the latest at the time of writing) and it's a bit like a 'director's cut'. I'm hesitant to recommend the unofficial patches for a first play through, as some of them introduced new bugs while getting rid of the old ones. If you wanted one for your first play through, I've heard that 1.8 is as close to bug-free as it gets.