Baldur's Gate 2.
Out of the box, the game was fantastic - some bugs, but none ridiculously crippling. Amazing characters with actual depth, quests/side quests that weren't pitiful gophering around, high level of control in terms of combat, rogues that *actually used* their rogue abilities to find traps/pick doors and locks... the list goes on.
But what sets it a league ahead of other RPGs was the modding community. The base game was gold already, but people released so many amazing mods for it, you could basically play it again and have a completely different experience. Removing the 1-stronghold-per-char limit, increasing frequency of random battles, adding new classes/items/shopkeeper npcs, and adding the option to skip the intro dungeon without losing loot/exp are just a few of the infinitybillion mods that I can remember offhand.
This isn't the same as, say, modding in Oblivion. Obliv was a piece of shit game that *required* a folder full of mods and addons to get rid of enough of the suck to make playing it bearable, not to mention fix the gamebreaking bugs that Bethesda never seems to care about. BG2 mods were just icing on an already amazing cake, not icing on a sawdust sandwich.
The combination of an amazing game out of the box and a plethora of great mods puts BG2 ahead of any other RPG.
Edit: While Morrowind/Skyrim/Fallout all have their strengths, particularly in the immersion side of things, they all fall far short of BG/NWN simply because of the focus on solo play. An RPG without a party is inherently a weaker experience than one with - it was one of my problems with Morrowind in particular. You could spec as a thief and get good at stealing great pieces of armor and sexy 2 handed swords, but as a thief, you sucked at using them. The warrior types can stare at the pretty equipment on the wall, but not steal them. In a real RPG, the thief would pilfer chests/pockets/cabinets/etc and distribute the loot to the rest of his *party*. (This is also one of the many problems with Dragon Age/every final fantasy game with a thief; the thief class is a waste of time, completely unable to pick pockets or disarm traps or any of that good thiefy stuff).