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.