That's hard to say, as it often depends on the mood I'm in.

I'd probably lean towards Pool of Radiance, with its relatively non-linear quest structure, tactical combat, and ruleset. Sure, the later Gold Box games made many refinements to smooth out the engine, but I didn't ever feel as though they quite captured the magic of the original.

And, while not technically an RPG, I'm a big fan of FRUA (Forgotten Realms Unlimited Adventures), which is a construction kit for Gold Box engine adventures. Straight out of the box, there's a lot of things that were missing (due to licensing restrictions, I believe), but the community has managed to put together quite a few hacks that make it a pretty decent piece of software. And, some of the modules that have been produced are a lot of fun.
 

Does Planescape: Torment count as old school? If so, I'll pick that for the amazing setting and storytelling.
 

Wizardry 1. No, really. Wireframe graphics and all...
 

Ultima 5. A gritty world with a significant moral dilemmas (Blackthorn's laws), and the feeling of true danger from the Underworld.
 

Another vote for PS:T here; also partly the setting and partly the story and characters.

If I had to pick another one... I'd probably go with Ultima VIII, since that was basically the first graphical CRPG I've played. (I haven't as yet played the earlier Ultimas, in fact, although I have played IX. I liked both VIII and IX a lot, despite what some people say about them.)
 

Might and Magic III-V: huge world, free exploration.

Is Baldur's Gate already "old school"? SOA is really memorable: great villain in main story arc, many diverse subquests with it's own stories, romances, tactical combat.
 

Nostalgia says Eye of the Beholder 2, though I can agree that the story was rather... no, I guess EoB2. There was a story, and there were lots of room in which to "inject" your own story.

(I also would like to drop a thumb up at Ultima Underworld and Might and Magic 3-5)

I *would* have put my vote at PS:T - but old as I am, that one isn't "Old-skool" yet ;)
 

I can't pick a favorite, because I enjoyed all the old RPGs. But since the early (turn-based) Ultima games and Pool of Radiance have already been mentioned, how about Wasteland? That's one of the very few RPGs I ever finished. Lots of fun!

And the two Magic Candle games were great, too. You don't hear so much about them these days.
 

I will second the Ultima 5 comment - the underworld is a nice touch.

Perhaps my view is clouded a bit by the excellent Lazarus remake though :)
 

didnt know if we were potsing here or the forum..

Ultima III, Wasteland, Magic Candle I, Demons Winter, Questron II, hack (the nethack precursor), Pool of Radiance, Bards Tale II
 

Wizardry and Bards Tale. Mostly because they were the first cRPGs that I completed. Dark Heart of Uukrul is also a forgotten classic - first rpg I played with a useful automap, plus it had excellent puzzles.
 

It wouldn't really hold my interest anymore, but Ultima V is probably the best classic RPG in my book. Even though I had played it after playing Ultima III and IV, there was such a sense of discovery and it made me get lost in the game. Plus, I loved the addition of a morality system.

Faery Tale Adventure also has a special place in my memory, mostly because of one encounter where I was lost in a dungeon and my character was about to pass out from fatigue. I ducked into a room and closed the door (not that I expected that to save me). My character woke up the next day, still alive, and I found the way out. Nothing too dramatic, but that was one of the most immersed gaming moments I'd had to that point.

Oh, and Legacy of the Ancients. I don't remember doing much but playing blackjack and delivering packages, but the world felt so big and I loved watching as my character grew in power and mastery of his world.