Baldur's Gate had a terrific final boss. Like most of the other big battles in the game, it was up against a party of NPCs, rather like your own party. Ok, you only had to kill Sarevok, but if you didn't kill at least some of the others you didn't have a chance. It was a ridiculously difficult battle, but it wasn't cheap: you had to really think tactically about how you were going to attack them, whom you were going to attack first, and what spells you were going to use. The sense of accomplishment afterwards was fantastic.
By contrast, Baldur's Gate II's final boss, while still very good, was slightly underwhelming in comparison. Once again, you were up against a group of enemies, which made for a big, challenging battle. The problem was that Irenicus himself seemed oddly weak, especially considering how powerful was earlier (you have to fight him a few times throughout the game). I just hit him with a Breach spell and he was at my warriors' mercy. And yet, while he was quite weak, you have to kill four very powerful demons to be able to attack him at all. So the battle itself was still as good as the one in BG1, but the character who was the boss was slightly disappointing.