I always found the atmosphere at Belthasar´s lair in 2300 A.D. to be one of the strongest in the whole game.
The horrible future surroundings combined with that mysterious piano track gives me the shivers every time.
It´s like a weird dream, and no doubt one of my favourite moments of the game, even though it´s also a bit uncomfortable.
I could listen to the piano melody all day and get the same feeling every time!