There are many classics to be played, good to hear you're going back a bit.
My reccomendations may be a bit older than most others, which well, should almost be a pre-requisite for the classic tag!
I'll stick to RPG's.
A few classics which I've found very inspiring over the years:
Ultima IV: Quest of the Avatar
Ultima VII: The Black Gate
Ultima Underworlds 1 & 2
Curse of the Azure Bonds (Yep, that 'old SSI RPG engine!)
Bard's Tale III: The Thief of Fate
Buck Rogers: Countdown to Doomsday
Buck Rogers: The Matrix Cubed
Eye of the Beholder I and II
Lands of Lore
Ravenloft: Strahd's Possession