I was going to say, hands down, Baldur's Gate, but it was already mentioned by Mechanized. For reasons he stated, I can't really fault any of the programmers of the older games, as there were limitations and you could only do so much without detracting from the game play.
But Baldur's Gate...ugh. I just finished that on Sunday, and boy, did I feel violated. *spoilers* First, the main boss is nothing more than a chick in armor. No demon, no Beholder, no other kick-ass, complex creature. Second, she took literally 20-25 seconds to beat. THAT'S IT!! THEN (it gets better), she drops to her knees, confesses the error of her ways, and tells the hero to proceed. The hero proceeds into a portal "far more dangerous than where he came from," and as shadow creatures approach him, the fucking game tells you to, essentially, tune into the next game to see what he's up against. And that's the end of the game (after the credits, of course). *end spoilers* Man, I was pissed. Pretty good game overall, though not exceptionally difficult (you were pretty much told what to do and where to go), but fuck that ending blew.
I'm glad to hear for a fact that BG2's ending was equally bad, as I was about to buy it for the PC just to "continue" my old game.
John