I was gonna say Ragnarok Online but I ended up getting sick of it (mainly because went from casual to a PVP/competitive player).

Jumping through all sorts of private servers with multiple guilds made me quit playing back in 2012.

Closest to an addicting game I can think of.

Games don't TRULY grip me on that level these days unless it's a fun online game or a REALLY enjoyable open-world single player ARPG type game like Assassin's Creed or Borderlands. I guess beyond those two - Overwatch maybe, Starcraft og, Diablo 4 on a few occasions when playing with a friend or close to leveling up.

Back in the day I could play online Black Ops, Halo 3, Gears trilogy for several hours a day and totally lose track of time.

Roller Coaster Tycoon, Pokemon Red, Sims, Minecraft, Terraria were also pure video game crack for me as a younger lad.

I have to say any good player multiplayer online game can take over my life so I stay away

I had this experience with online multiplayer games. Years ago it was text-based MUDs, where your input and the games output is text and you play it on a server. These games are community generated and you play together with others. This was the predecessor of MMOs. I got addicted to that, had to check in only because I could communicate with others and if I felt like it make a quest or fight monsters. Also to explore the worlds.

I have it a bit with Elder Scrolls Online, again very similar, as I said MUDs are kinda the predecessor to MMOs. But there is a difference, in ESO I barely touch the online stuff and multiplayer content, I love to explore the world and single player quests. It is just - that there is soooo much of that, Tamriel is massive and it also contains multiple planes of Oblivion. The daily login bonus is actually evil. So I stop playing for months at a time.

And I actually stepped for a while into Fortnite Battle Royale. It was fun, it is pretty easy as most players act like absolute morons, my reactions are not up to par for a fast shooter, but it is easy to outclever most other players which let's me make the TOP 10 in around 50% of cases. Completing the battle pass that it itself pays for the next was doable - but I had to play daily. Once I realized this made me stop playing anything else I stepped away from it.

EDIT: There were other games that felt addicting while playing, but many other games have a defined end or a point where I feel like I reached a point of fulfillment. So I stop them naturally. These above are potentially endless or at least so vast it can take years.

10,000 hours in a moba is an insane amount of time, let alone 20,000. An average match in Dota 2 is longer than league maybe around 1 hour, so he played like 20,000 matches? Win rate be like 10,000 wins and losses if matchmaking works well. Just to be clear 20,000 hours is more than 2 years and 3 months of playing an intense game. Dota 2 has only existed since 2013, so out of 11 years he has played dota 2 for 2 years, sleep maybe 2 or 3 years? Job? I think only pro level players would be above 20,000 hours, so I hope your buddy is at least trying to go pro, and it is at least his job.

Actually found 7 accounts with more than 2 years played, is he one of them?

https://www.dotabuff.com/players/duration

Top 100 have around +10,000 hours maybe up to top 500.

Ragnarok online
WOW
Age of Empires II
SimCity/TheSims
Monster Hunter in general, specially MHWorld/Iceborn 

Lowkey Elden Ring as well, I did 4 consecutive runs and had to force myself to not start a 5th for the sake of moving on to other games... and yet, im just waiting for that DLC to start a new one a couple of days before. 

All Blizzard games between 1998-2016 for sure other addictions were Bungie’s games starting with Halo CE but ended with Destiny 2 that I now have regrets lol. Then there’s Age of Empires 2, 3, and Mythology.

Panzer General II, Fantasy General and Master of Orion II: Battle at Antares in my youth. I remember making a 16-hour session of Fantasy General one day/night and quickly hurried to bed when I heard that my dad was waking up so he wouldn't notice...

Overwatch and Call of Duty. Both series I put several thousands of hours into and would rarely play much else. Pretty much dropped both of them over the past few years and now focus more on single player games I have missed over the years (and those still releasing). Will get back into both eventually though!

Pretty much every strategy game I've played. Don't know what it is but when I play strategy games 10 hours feel like maybe 2. When I play other games time flies much slower as in strategy games.

I never touched an MMO because I know I would waste my life and wouldn't do anything else anymore as to play that game.