[Kayless]

Depth of game play eh? For me that would be the Sims with all it's expansions installed. You start off by designing your own little people. Then you've got make sure your little dudes eat, sleep, get entertained, go to the bathroom, and talk to each other. You also have to train them in a variety of skills if you want them to progress in their jobs so you can buy better stuff for their house (which you can design yourself). Add in convoluted romance options, pets and their training, celebrity ranks, having children, going on vacation, throwing parties, pests that hang around your house (skunks, obsessed fans, etc.), unusual items you purchase, not to mention all the customized crap you can download online to make things even more involved. The Sims goes as deep as you want it.

Of course Fallout 2, with its non-linear "do whatever the hell you want" style of game play was cool as well. Its one of the few games that actually lets you play an honestly evil character. Become a slaver (and sell your NPCs into slavery), a child killer, a grave robber, a made man, a porn star, or just some freak that likes to butcher entire towns. Of course you can just as easily play a traditional good guy (or something in between). Fallout 2 also actually paid attention to your stats for more than just combat purposes. Want a fun challenge? Play as a character with an INT of 2 or 3. The dialogue options are hysterical (especially when you talk to Tor the village idiot of Klamath).

For depth of plot I'm going to concur with my esteemed colleague and go with Planescape: Torment.