Oh man, this is a tough one. I've had a lot of obsessions potentially as a kid. More recently though, that went to a game called Stellaris. You might not have heard of this game, but it's by the same guys who made Europa Universalis and Crusader Kings. It's a grand strategy/4x game set in space, with the basic idea being that you forge your own narrative. That game I got recently, and I sank hours into it. I'm about 60 hours in, still only my first game. I don't usually buy expansion packs for games, mainly cause I'm lazy, but this game was one of the few that I actually bought all the expansions for, just because of how much I liked it. I've obsessed over this game quite a bit, since it really does feeling like writing a sci-fi history novel of sorts. I'm not obsessing so much now, since that first game I played is sort of ending, but I get the feeling I'm gonna try something in a new game, and it's gonna be crazy fun again.

Other than that, there'd be Battlefield Bad Company 2, which I was for some odd reason, really good at, and I played it to the point of mastering each class, a rarity for me in online shooters. In the old days, I liked the original Star Wars Battlefront series a ton, especially couch multiplayers with friends. Good times. 

That's what I can remember off the top of my head. There's probably more though, tbh, since I've just been gaming so much. Not as much these days tho, since I'm just busier now.

Probably it was Open Transport Tycoon Deluxe a few years ago. I managed to grow the little cities almost to make the map into one single giant city. As far as I remember it took weeks, 8-10 hours of playing per day. And it was not the only "mission", I also played on several other maps where I focused on the clever rail junctions more.

DIABLO 2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

nuff said

I played Quake 3 DeFrag since it was released ~2000 and was very addicted to it (Progressive Trickjumping) - played it for many hours each day.

Its still unmatched in terms of physics which was the core of defrag mod and all trickjump capabilities.

Playing the same map over and over again:

- just to get a few seconds/milliseconds faster

- complete the map at all (some maps are pretty hard)

- reach an near impossible edge after thousands of attempts

Today i still occasionally fire it up and play it until my eyes are red like hell - which i get after half an hour nowaydays (I am getting old).
 

I will for sure still play this even when i am old and grumpy - as long as my eyes will work.

I remember it very well the feeling i had after finishing cos1 white (very hard strafe pad map) for the very first time.

Of course i made videos as well: