The Golden Mammoth of PC games
Every now and then, in every area of the human sphere, comes a masterpiece that influences the very lives of those who experience that masterpiece. Baldur's Gate 2 IS (along with Deus Ex but thats another story) that masterpiece - they are the standards on which I personally build my opinion on games. Its one of those games that you obsessively play until you finish it - provided you are not impacient with the relatively slow pace and learning curve (if you have no idea how the D&D combat system works) of the game and shelve it right at the beginning - and when you finally do, you just get sad and want to live all those beautifully woven storylines, all the carefully polished quests once again. If the combat system is something new to you, whatever you do don't give up. Keep trying to "pick the game up" and in the end it's guaranteed that you won't regret it.
Even though its an outdated game engine, it still isn't unpleasant to your eyes and is the pinnacle of the great engine that Infinity was. The sound effects are not on-par with todays wonders as well, but they fit right in and don't mess your experience. Voice acting is great though, gotta love Minsc's comments or say, Edwin's. It really makes the characters alive.
I've finished BG 2 and BG2: ToB near to 10 times, and its an absolutely huge game. I'm an avid RPG'er since Ultima VII: The Black Gate and in my opinion, PC RPG history began with the old text adventures but Baldur's Gate 2 was its "year Zero". There's what happened before and what happened after.
To sum up, BG2 shares the throne of single player RPG with Deus Ex, and IMO only Oblivion and perhaps KotoR even came remotely close.