I played almost exclusively Warcraft 3 online for 8 years...I have no idea how many hours I put in to it.. but I'm gonna guess, and I think this is on the lower side that If I played for 4 hours per time, 200 days a year for 8 years - that would be 6400 hours.. but honestly, it could very well be much more than that.

Really can't tell. I played a lot Tekken 3 on PSX and Tekken Tag Tournament on PS2. In the near past I'd say Mario Kart 8 and Mario Maker on WiiU with more than 500 hours.

I can’t be sure. There are three contestents; RollerCoaster Tycoon 2, SimCity 4 or Anno 1602. So all old, endless PC games, which I played religiously since release though the last couple of years finally less and less.

I think RollerCoaster Tycoon 2 is most likely the winner. Sadly none if these keep time, and that wouldn’t have been correct anymore after so many computer-swaps anyways, but I think 10,000 hours isn’t an unrealistic claim. However perception of time is different than reality, so it could be a lot less, but still definitely several thousands of hours. No console game ever came close. Of the top of my head Mario Kart 64 is likely the winner there, which may have also reached 1,000, but it’d be barely.

For certain I know I've spent 286 hours in Monster Hunter World and 224 hrs in Monster Hunter 4 Ultimate
But for around 4 years I played Ragnarok Online and for around 8 years I played World of Warcraft almost religiously, so I imagine ive spent many more hours in those games.

I don't know how long (wasn't tracked at the time) was the longest among them, but all of the following are easily above 100h, some probably even above 1000h:

PC:

Supreme Ruler 2020
Hearts of Iron III
Victoria II
Fantasy General
The Settlers
The Settlers II
Battle Isle III
Civilization IV
Sid Meier's Alpha Centauri
Eador: Masters of the broken World
Baldur's Gate

Consoles (including handheld):

Dragon Quest VIII
Dragon Quest Monsters Joker
Super Smash Bros Brawl
Mario Kart Wii
The Legend of Zelda II: The Adventure of Link (damn that great palace and it's approach!)
Tetris
Soul Calibur III (and damn you, memory card erasing bug!!!)

Smartphone:

Hay Day
Dots Hero II
Wicked Lair

Absolutely no idea. Probably Pro Evolution Soccer 6 with a few 100 hours, but that is just what comes to my mind right now. Pokemon Silver was around 400 hours so that might or might not beat it.
Current gen it's Dragon Age Inquisition with only 85 hours. There's too much stuff to play nowadays.

It's either Age of Empires II or FIFA07.

25.000+ Lotro.
10.000+ Aion, Skyrim, Roller Coaster Tycoon 2.
5.000+ Smash Melee.
1.000+ Pokémon Blue/Gold/Ruby.
500+ Just way too much.

It's hard to say. The 1,000 hour ~ 2,499 hour range is what I chose, as I know for a fact I have over 1,000 on hours on several games. Steam indicates that I have somewhere around 1,100 hours on Path of Exile, if I recall correctly.

Just checked, and Path of Exile is at 1,157 hours.

1. Path of Exile: 1,157 hours
2. Torchlight 2: 464 hours
3. Dragon Saga: 281 hours. Formerly Dragonica, a grindy, Korean, F2P MMORPG which I had originally played many years ago with my wife back in the days when MMOs were a dime a dozen. I found it on Steam a couple years back and started playing for old-times sake. The game is practically dead now, and it is constantly having server problems. It was down for several days in a row I can recall.
4. Skyrim SE: 195 hours. I also have 121 hours on the original Skyrim release, so if I bundle them together that would put Skyrim at 316 hours.
5. Fallout 4: 160 hours

Of course, this is only Steam, which is a fraction of my gaming. I play WoW usually for two months at a time every couple of years, and I would estimate that I have close to 100 days of playtime, which is the threshold for the next tier in the poll but I am unsure, and my best guess would be something around 90 ~ 95 days played. I have no idea how many days of playtime I have on FFXIV. If my sub was active I could check, but my best guess is 35 days.

Smash Bros. Melee is also a very top contender. I felt like I played it everyday for years during my childhood, but more accurately it's probably been around 500 ~ 700 hours.

I have about 1,000 hours in Destiny, over the five-year run between the two games. About 700 in Destiny 1, and around 300 in Destiny 2. I don't think I have more than about 150 hours in anything else. Probably The Witcher 3 and red Dead redemption 2 are in the 100 hour plus category. I might have over 100 hours in the original Halo, over several playthroughs.