Three main candidates:

Warcraft III

Final Fantasy VII

Final Fantasy X-2

I was so into Warcraft III a number of years ago. I played it so much, made so many pointless maps, eventually I stopped playing when DotA rose to the stars because I couldn't get into it.

My playtime on Final Fantasy VII is through the roof since I found qhimm and its tools. I love letting my creativity run free with them. And that's on top of the ~6 times I'd already played through the original game in the first place.

Final Fantasy X-2 is probably my most played FF and I suspect it being the game out of these three that I spent the most time PLAYING. I spent a lot of time in the editors of WC3 and FF7 so that adds up, but the actual ingame time doesn't really compare to FFX-2. Counting NG+ runs I probably played through the game 15+ times, not counting them I think it was 5 times, and every one of them was easily three-digit-playtime. And then there's a considerable amount of time that isn't recorded on those playtimes due to Game Overs or when I did challenge battles and didn't save afterwards. There's so much you can do in this game, so many ways you can battle enemies, I just love it.