I can't be fully sure, but I'd guess Final Fantasy VI.

"World of warcraft, 120 days playtime."

I believe that it is more common than most people think. I know a guy who was playing WoW during 90% of the time he was awake since its release. He just left the computer to go to the toilet or to get some bread and butter to eat while he continued playing. He was playing so much that his parents forced him to seek help for gaming addicts.
It's not like he is the only one out there.

I have played a lot of morrowind, mostly because of the TES editor.

This gen:

Zelda Wii: 35 hours.
Mario Galaxy: 32 hours.
Resident Evil 4: 24 hours.
Paper Mario: 34 hours.

Ninja Gaiden Sigma: 120 hours.
Resistance: 15 hours.
Heavely Sword: 5 hours.
Mass effect: 46 hours.

I can go on and on. I'm easily one our sites most balanced, all-around hardcore gamers.

FFX, GT2, GT3, GTA2, GTA3, GTA:VC, AoE3, over 300 hours on each. This gen its RFOM and CoD4 with about 200hours each.

Usually it's strategy games that tend to just eat my time, and these are mostly on the PC. Civilization (Civ 2 the most) and championship manager are those that come to mind.
I have no idea how many hours I have spent on these games though as I've played them for years and years.

very easy answer for me: HALF LIFE death match online. i played that game for over 2 years so i played it atleast 5000 hours in total

One of the Final Fantasy's...7 or 8 maybe...7...either way, well into hundreds of hours because of replays and such alike.

aniaml crossing on ds, i got on launch day and have played it every day since.

World of Warcraft and Super Smash Bros. Melee.