All of the above, really. And it wouldn't even be particularly expensive... But of the options listed:
Baldur's Gate 1&2 (and the expansions, of course) first and foremost for the sheer overwhelming amount of content. Great writing, excellent characters, fun gameplay. These are, overall, the best of the bunch. Planescape has a better story, Icewind Dale has more tactical combat, etc., but Baldur's Gate saga has it all and is a well rounded investment that can make you spend hundreds of hours.
Planescape:Torment for one of the best written and original RPGs ever. The gameplay and combat is a bit shallow at times, but the game is a freaking interactive book and I mean that in the best way possible.
Fallout 1&2 come in third place as another pair of well rounded games. You can kick them up to second place if you love post-apocalyptic settings. Free, open world, interesting locations to explore, tons of (dark) humor. Graphically somewhat weak, but not too bad.
Icewind Dale 1&2 in fourth. Featuring the same engine used in Baldur's Gate series, these games focus more on combat and tactics and less on the story and choices. This doesn't mean that they have no story (they do, and it's good), but they are far more linear.
Diablo 1&2 in a faaaaar fifth place (imagine a wide gap between 4th and 5th). Get this if you want to click stuff to death. In fact, you might as well skip D1 and go straight to D2. Save yourself some pain. Diablo was great for its time, but action games don't age as gracefully as proper RPGs and D1 is a bit painful to play nowdays. D2 is a decent (if a bit archaic) hack&slash with varied classes and a popular multiplayer mode.