Wizardry 1. Ultima 2 came close, but Wizardry 1 really was it. I was shit at the old platformers, and a 5 year old nerd that deliberately got up after my folks went to bed so I could read the intro to programming manuals that came with our Apple IIe - not that I remember any of that siht. I'd then be plainy Wizardry at least 2 hours a day, every day, and for a while I'd be playing it another 4 hours at night, until my folks were told by my teachers that I was falling asleep in class. They monitored me to see what was going on, and caught me sneaking down to the computer room when I thought they were asleep. Fuckers. I'd even got into the habit of setting my alarm for 1am by that stage.
You can imagine how difficult that game was as a kid, but also how utterly amazing the idea of it was. The idea of playing a repetative donkey kong clone, with one or 2 screens of jumping puzzles repeated, or this incredibly large (for the time) 3D dungeon with traps, mapping paper, party building..just amazing. Ultima 2 was actually more mindblowing, with its open world and implementation of time-travel, but it didn't have the great party and combat mechanics of Wiz 1, and as a 6 year old I was always thrown by the sudden change in gameplay from early-game survival to late-game puzzle-solving and exploration. Basically you had to scrap to survive until you got a boat - then you'd sail around blasting everything and grinding levels and stats before getting uber enough to get a plane and a rocket, and explore space, etc. Never did figure out how to defeat Minax, but I'm sure if I did it now I'd be disappinted by how simple it all seems today.
Part of what I loved about Wiz 1-3 was the way they balanced a complex stat-driven set of rules with the player only seeing the outcome of their actions. Even discovering what stats and builds were necessary to unlock the hybrid classes (samurai, ranger, bishop) and uber-classes (lord and ninja - neither hybrids nor ubers could be selected at start, but needed certain advanced stats and builds to make them avaialble as a class-change option) was a lot of fun. I never discovered the precise means of unlocking ninja (I WAS 5-7 when I played it) or Lord, though I occasionally got them by accident, until the later Wiz games many years later. I suspect it was actually easier to unlock lords and ninjas in later games, and that they were nerfed slightly (they were nerfed massively in Wiz8 when you could just start as them - before then ninjas could tank as well as do uber damage, and lords could tank whilst having genuinely powerful cleric abilities).
Similarly, you wouldn't know the exact class modifiers. You'd get the description that samurais are really effective fighters, but you wouldn't know exactly what that meant (from trial and error it meant they got lots of attacks and were as good on offence as fighters, but without the defensive / armour/hp tanking ability).
Man i loved those games. But I can recognise that it's just nostalgia now - I couldn't go back, or if I could I couldn't justify it to someone who hadn't grown up on those games (unlike, say, the great late 90s rpgs).