Excluding avatars like the Villager in Animal Crossing and the protagonist of Stardew Valley, it's easily Mario or Luigi. When I can pick between the two, I play as Luigi more.

Mario or Mega Man I think.

2020s - Geralt, been through Witcher 3 multiple times.
2010s - Link, got like 500 hours of Breath of the Wild before Coronavirus.
2000s - Villager, Animal Crossing franchise.
1990s - Probably Donkey Kong, Sonic, or Cecil… I played other RPGs more, but FF6 has no main character. Many PSX RPGs I got toward the end of the decade and played at least as much after 2000.
1980s - Probably Link again… Maybe Mario.

Alltime - Villager, Animal Crossing franchise

Across games, Mario by a landslide. Super Mario 1, 2, 3. World, 3D World, Sunshine, 64, Odyssey, New Super Mario, Wonder... Paper Mario, Origami, Color Splash, 1000 Year Door, etc. No idea how many total hours, but christ it has to be a stupid number. Oh and Mario Kart.

Single game, Link in BotW.

8/16-bit 2D Mario, easily. I can play those games back to back for the rest of my life and not get burned out in the least.

My old main from Wow must be a 1000+ hours, even remade them for Classic last year.

I imagine runners up would be Link (400+ hours), Geralt and Joker (250+ hours) and maybe some starter Pokémon if that counts, if not then the Trainers from Gen 2 and 3 for sure (300+ hours each I would guess). Since you can change characters in Tales games I don't think any of them reach quite those number, though Lloyd might be close. Cloud would also hit those numbers if you couldn't switch in the remakes.

Either Luigi or Donkey Kong.

Shit, I forgot about Fox McCloud, must have played over 1000 hours as him with countless runs of Starfox SNES and N64 as a kid.

Mario and Link are most definitely leading the list on my end.

Warrior from diablo1,thousands of hours