Most recently, at 400 hours and counting (340 in steam, plus the hrs I played offline), I think it is Crusader Kings II.

All time, it would perhaps be Tekken 3, Diablo II, or AoE 2, all of which occupied me for years.

Yeah it was no lifer for me at the time. I would consistently pull all nighters during the summer and weekends and that alone is playing from like 3pm to 7am, aside from food and bathroom breaks. At times I was crazy enough to do double all nighters. In fact I did indeed play more then sleep at the time.

Just like the OP, Age of Empires II and The Conquerors Expansion.

Probably osu!, this free rhythm game. When I used Xfire for game time tracking (and before it was shut down), I had clocked about 2000 hours for the game. Now, I'd be surprised if it wasn't above 5000 hours. It's just so easy to waste relatively short times on osu! every now and then! And no, I'm not a fan of rhythm games. I was reluctant to try the game but someone heavily recommended it, and here we are...

Minecraft is the only game that I used to play every single day for hours on end with my friends, so it's pretty much the only possible answer for me. I don't play it anymore, but I really wish I could see how many hours I've played it back then, I'm pretty sure it'd surpass 2000 hours.

Super Smash for Wii U - 1000 hours. Many of those are idle time when company's been over and we've all had a few and passed out, lol

Zelda Breath of the Wild - 400 hours. Do I even need an explanation? Just chatting about it here is making me want to go play it...

Xenoblade Chronicles X - 500 hours. Flying around in a giant robot is addicting. Who knew?

Metroid Prime - 200 hours. One of my favorites. Just so great to run around and log data while 100%ing.

Probably more where I haven't realized I've been playing for so long.

Im not a guy that will spend thousand of hours in a single game, but if i had to choose:

The Witcher 3

Kingdoms of Amalur: Reckoning

I just did every single thing in the game. Masterpieces IMO.

Call of duty modern warfare 2. I played that game everyday for 2 hours plus a day for 4 months. Super addicting game and it did not have stupid loot boxes. Fair and fun game.

It’s either Gran Turismo 5 or Final Fantasy X. GT5 I probably put over a thousand hours into in the process of getting the platinum. I probably spent over a hundred hours just on the Vettel challenges alone. FFX (the ps2 version) I have have 2 separate perfect save files. The grind to getting a perfect sphere grid in that game is insane. Unfortunately I no longer have the memory card that had the save files but I’m pretty sure I put over a thousand hours into it.

WOW and no other game will ever come close.