Absolute earliest video gaming memory: some unknown Atari game. I can't remember as much about the game as I can the room in which I played it, or the TV it was connected to, but then again I'm pretty sure I was 4, so.. there's that. Pretty sure it involved dropping bombs on battleships, but that could be wrong.
Earliest detailed video gaming memory: Sonic the Hedgehog II on Sega Genesis. Also served as my introduction to cheat codes, lol.
First game to consume my life and make me obsesively take detailed notes because I didn't have a prima guide? Shenmue for the Sega Dreamcast. Somehow I'd made it through Ocarina of Time without incident. Same with Majora's Mask. But in Shenmue, every NPC had their own lives, schedules, homes, weather responses, etc... And while, by and large, the game developers did try to either point you directly to special NPCs you'd need to talk to for quests and whatnot, or they'd try to make them stand out enough that you'd go up and talk to them on your own; As a kid who'd never experience this level of Open World gaming, this was fantastic, and I remember being thrilled that everyone was so detailed, voice acted, and real feeling. Nostalgia goggles, help, because by today's standards the game doesn't really hold up, lol.