You know it! LOD introduced me to the world of RPG's!

Sega Megadrive 16Bit : Sonic

Ps1: C&C Red Alert 1

XBox: Halo 1

Xbox 360: Halo 3

Xbox One: Battlefield 1 but i think Battlefield 5 will overtake it soon.

Pc: Hearts of Iron 2 or Medieval 2 Total war not sure i got this games 12-14 years ago and still play it from time to time

Hmm this is hard.

I'll seperate handheld and home console. I'll also do a PC section. :)

Handheld
Gen 4
Gameboy
Pokemon Yellow (Super Mario Land is a close second)

Gen 5
Gameboy Colour
Pokemon Gold

Gen 6
Gameboy Advance
Yu-Gi-Oh: Worldwide Edition: Stairway To The Destined Duel

Gen 7
Didn't have anything this gen

Gen 8
Ethier Pokemon Y, Samurai Warriors: Chronicles or Fire Emblem: Awakening, not sure at all.

Home Console
Gen 4
Mega Drive
Sonic The Hedgehog (Moonwalker as a close second)

Gen 5
Playstation 1
Gundam Wing: Battle Assault 2

Gen 6
Gamecube
Super Smash Bros. Melee

Gen 7
Wii
Wii Sports (Mortal Kombat : Armageddon is a close second)

Gen 8
Wii U
Super Mario 3D World

Hybrid
Gen 9
Switch
The Legend Of Zelda: Breath of the Wild

PC
Windows 95/98
Age of Empires

Windows XP
Star Wars Empire At War/Forces of Corruption

Windows 7
Total War: Shogun 2 or Skyrim, not sure.

Windows 10
Total Warhammer 2

NES era- Super Mario Bros 3.
GEN-SNES era- I spent a lot of time on Sonic 2 and Chrono Trigger but thinking about it. I think the game I most played was Mortal Kombat 1-2.
PS1-N64 era- Zelda Ocarina of Time (Honorable Mention to Final Fantasy VII)
Dreamcast-PS2 -XBOX-GC- Surprisingly while not my favorite game it was probably Speed Devils Online for the DC (my high school grades suffered bad) because of my addiction. Honorable mention-Halo
PS3-Wii-360 era- Skyrim honorable mention Gears of War.
PS4-XBOne-Switch era - Breath of the Wild honorable mention (Horizon Zero Dawn)

I can't say for certain with some of these, but this would be my best guess:
Gen3: Super Mario Bros. 3 most likely. Other possibilities would be SMB1, Mega Man 2, Mega Man 4 and Kirby's Adventure
Gen4: A few possibilities. Donkey Kong Country, Mortal Kombat 3, Final Fantasy VI.
Gen5: Final Fantasy IX, Tekken 3 or The Legend of Dragoon.
Gen6: Final Fantasy X, Tekken 5, Super Smash Bros. Melee or Soul Calibur 2.
Gen7: Super Smash Bros. Brawl without a doubt. After that maybe Valkyria Chronicles.
Gen8: Bloodborne or Dark Souls 3.

I left out PC games since you can't really split them into different generations.On PC I've probably spent the most time on Red Alert 2, League of Legends and Total Annihilation, although there are a number of other possibilities as well.

Gameboy: Yugioh Dark Duel Stories or Pokemon Crystal.
Gamecube: Smash Bros. Melee.
Gameboy Advance: Fire Emblem Sacred Stones, Castelvania Aria of Sorrow, or or one of the many Yugioh GBA games.
DS: Advance Wars Dual Strike/Days of Ruin, Castlevania Dawn of Sorrow/Portrait of Ruin/Order of Ecclesia.
Wii: Smash Bros. Brawl.
Xbox 360: Halo 3.
PS3: Resident Evil 5.
3DS: Fire Emblem Awakening
Wii U: Smash.
Xbox One/PS4: Overwatch
Switch: Pokemon Let's Go Eevee.

Hunders of games... per generation? Pff no. When I had time, I didn't have money, and now that I have money, I don't have time. Anyway, let's get back on the topic:

• PS1: Hard to tell, but my guess is Driver 2: Back on the Streets.
• PS2: I'd be somewhat surprised if the answer wasn't Dynasty Warriors 5.
• PS3: It's probably Final Fantasy XIII. Getting the platinum trophy took a while... That said, Assassin's Creed II too is a plausible candidate.
• PS4: I'm surprised if it's not Assassin's Creed: Odyssey.

And then, of course, there's PC, which probably has some of my most-played games. I doubt any half-recent console game holds up to my 310 hours in Rocket League or my 288 hours in Grim Dawn. And then there' osu!, which doesn't (or at least didn't use to) track time, but back when Xfire was still around, it was already at about 2000 hours... and it's got to be way more now. I doubt any games is ever going to pass that one for me! And then there's of course olders games from a time when time tracking wasn't around or it's been lost (Xfire, I'm looking at you...). Command & Conquer: Red Alert 2, Wolfenstein: Enemy Territory, and especially Titan Quest probably have some pretty high numbers. If we count Titan Quest in the 7th gen, I doubt Final Fantasy XIII has any chance against it.

I guess I can't do games on PC or other computers since they didn't have generations. On PC its Final Fantasy XI, on mobile its Dissidia Final Fantasy Opera Omnia.

Gen 2: River Raid (on an Atrai 7800 through backwards compatibility)
Gen 3: Kirby's Dreamland
Gen 4: Donkey Kong Country 1 or 2
Gen 5: Final Fantasy VIII
Gen 6: Final Fantasy XII or maybe Smash Bros Melee
Gen 7: Final Fantasy XIII
Gen 8 (including Wii U and Switch): Final Fantasy XV

Handhelds
GG/GB/GBC: Donkey Kong Land/Tetris DX/Link's Awakening
GBA: Final Fantasy I&II, IV or maybe Golden Sun: The Lost Age
DS/PSP: Animal Crossing: Wild World
3DS/Vita: Animal Crossing: New Leaf

What this tells me is that for most of these consoles, the top played games were also among my favourites or they were rpgs that often last a long time anyway. The arrival of Animal Crossing is a notable exception that gets the most play simply because it encourages you to play a little bit every day, that all adds up and I can see definitive proof on 3DS that Animal Crossing has much more playtime than anything else (about twice that of Smash Bros 3DS in second place).

NES era - Super Mario Bros. I don’t think anything came close, but if so, maybe Tecmo Super Bowl couch sessions with my brothers 

SNES era - Super Mario World or Donkey Kong Country 

PS1 era - Three games come to mind: Gran Turismo, THPS, and MGS. Probably GT

PS2 - easily SOCOM II. Closest second would be PSO on Dreamcast 

360 - CoD4 or World at War. Search and Destroy with work buddies

Xbone - Dead by Daylight. I think this gen was the gen of asymmetrical MP for me. Two of my favorite games were Dead by Daylight and Evolve.

Gen 4: Super Mario World
Gen 5: Loz: Ocarina of TIme
Gen 6: FF X
Gen 7: The Sims 3
Gen 8: Battlefield 1 or Destiny