Took me a while to realise it was bestselling on GOG (the list gave it away, but until then...).
Of these, and considering not only GOG purchases, Dungeon Keeper 2. Just not my kind of game.
Surprisingly enough, looks like I have at least 11 of the top 15 (not sure about Sid Meier's Alpha Centauri; if own it that would make 12). That's when counting some version variation (for example, I only have the original BG, not the Full Saga).
Edit:
Counting only things I own on GOG, Planescape: Torment would be the first. Why? Because I already own it on physical and not planning to replay soon.
Anyway, top 15 list ownership:
PS:T - physical
BG2 - GamersGate, Google Play (Enhanced Edition)
NWN - GOG, physical
DK2 - don't own, no interest
Theme Hospital - physical
BG - physical (only base campaign)
Alpha Centauri - not sure, I think I have a physical copy
NWN2 - GamersGate (complete) and physical (only original campaign)
HoMM3 - physical
Icewind Dale - don't own because combat oriented
Icewind Dale 2 - don't own because combat oriented
ToEE - physical
Witcher - GOG and Steam
Witcher 2 - GOG
Arcanum - GOG
Maxvorstadt: Oh, I forgot the reason why I don`t own Planescape Torment. It`s because I don`t like this D&D universe and the rules of it.
What do you mean by "D&D universe"? Planescape is quite a different setting than The Forgotten Realms.