Leaving aside the Renaissance era RPGs (Infinity Engine games, fallouts, arcanum etc) I spent the most time with Daggerfall, my favourite elder scrolls game. It seems like that was the one where every big idea of the series was born. Vampirism and lycanthropy, horses and carts, house and business ownership, guild membership and heirarchy, all of these mechanics were either introduced or refined in Daggerfall.
The game was ahead of its time. It probably has the biggest world in the history of open worlds. Yes, it's procedurally generated, but that's still an achievement given the hardware limitation from the time of its release. Even to this day however the game is also unfamous for its bugs. It was almost unplayable when it was released.
Also, big shout out to the library of classic RPGs from Jeff Vogel at Spiderweb Games. I'd love to see more of them make the jump to android.
I never played any of the Spiderweb Games. Didn't like the art style.