8bit era:
Maniac Mansion. Mostly playing arcade and sports games at that time: Still remember seeing that on a school friend's Commodore 64 and being all excited about a game with "real people" talking and walking around an actual place you could relate too... even chasing you... unfortunately, I had an Amstrad CPC at that time. (Actually still got a C64 after the disk drive went bust much later).
16bit era: Probably
Ambermoon on Amiga. It's the first beefy RPG I spent a whole lotta time on. It wasn't merely an RPG to me. But a foreign world to escape to... helped by the map that's shipped with the game. It also had texture-mapped 3d dungeons you explored real-time in first person, which at that time was fairly novel still, in particular on Amiga computers (combat was a seperate mode switching to a 2d turn-based screen).
PC:
Thief. As a recent retrospective on Looking Glass Studio's other cult classic, System Shock, had it: "We were trying to build the holodeck." You see, Looking Glass tried to bring the agency roleplayers had in a pen&paper session likewise to the screen, ever since their first game Ultima Underworld. Only that they did this by different means than copying pen&paper stats and to-hit rolls. Like systems interacting with one another, which can be manipulated (which BG3 actually has too, like the elemental combat systems, physics etc.)
True to LGS "holodeck" ideal however, Thief also sported a really minimalist UI and HUD too. Nothing was supposed to remind you that you were just playing a damn game. Thief has one mission dumping you into a series of crypts and caves and tombs, with barely anything to go by but a vague in-universe map at your hand (at one point, even the game's character just notes down a "WHERE AM I"? on it.) Then you suddenly HEAR the item you came looking for -- and can navigate the crypts by its 3D sound, which still holds up. In 1999, that looked (and sounded) like the future of gaming to me. Everybody's talking Virtual Reality these days. But LGS tried to do it back then. All with software.
Unfortunately, it never much evolved from there, as nobody's picked up Thief and completely RAN with its ideas -- it's not that this had ever been the perfect execution of those ideas, you know. And aside of visuals, things often times regressed. As so many first person games CONSTANTLY remind you that you're just playing a game with all their markers and intrusive UI and HUDS, tutorials, cramped corridor levels (or open world theme parks) etc. to the degree that even a three billion Dollar holodeck wouldn't save them.
(There was one exception. Which was Trespasser, a game based on Jurassic Park. Made by former Looking Glass staff such as eventual XBox designer Seamus Blackley, it took the whole VR idea a tad further -- a tad too far perhaps. The game had no HUD whatsoever and also lets you control the characters arm. And it also was overly ambituous, performed like poo even on the best machines and not much fun to actually play. But yeah).
