World of Warcraft by a huge margin. A couple of years ago, I went through all my characters, and added the /played time up. I think it was already over 10k back then, and I've played a bunch since then. Also, I often delete characters, so I can't count those. My sub runs for another 10 days, so I might take the time to check again.
Next is probably Diablo 2, but that was 20 years ago, when this stuff wasn't really automatically tracked.
On Steam I have two idle games at 800 hours, FF14 650h, both Nioh games 600h each.
I played Wow for the first 3-4 years after its launch.
I had a part time job back then, but otherwise I was playing wow. At some point my /played time passed a year, after that I refused to look. I don't know if it ever got to two years, but I fear it may have...