For me it was Baldur's Gate 2 without the expansion. It was the multi-CD version and it is the instigator of my current RPG addiction. After losing the discs I was fortunate to find a boxed-set for sale which included all the Baldur's Gate games plus expansions, all the Icewind Dale games plus their expansions AND Neverwinter Nights 1 and it's expansions. Still have that, but I now have GOG versions for most of these games as well as the brilliant Planescape: Torment, which I had only been able to play recently and still haven't finished, unfortunately.