Which RPG on GOG would you say is the best when you put story, dialog and atmosphere aside? (so just based on gameplay, mechanics, stability, audio/visuals, character creation, etc.)
I think people are missing what you're asking for here. I think I get it though. A game like Planescape Torment would be high on most lists but due to it's reliance on story, dialog & atmosphere for this list would not.
I'd think old school dungeon crawlers would be high on this list. Legend of Grimrock only has one real tileset, and has virtually no story. But it has solid old school mechanics, good graphics, and a pretty robust system of character creation.
And while I've never learned how to play the games, Realms of Arkania have the most in depth RPG mechanics I've ever seen. I've played RPG's most of my life, and I've yet to figure out how that thing works.
Temple of Elemental Evil with the Co8 patch gets a nod too. The story (...what story?), dialog and atmosphere are no good. But it has the best representation of what D&D is period. No other game comes close. And with Co8 most *most* of the bugs are ironed out, but you run into some here or there.