The lights are on
Final Fantasy VI...That is all.
Probably Pokemon Silver... I put at least 250 hours into that game...
Oblivion, followed by Pokemon Gold and then Timesplitters: Future Perfect. :p
Just Cause 2. I got really addicted to it for awhile.
Pokemon Pearl. 309 hours and 25 minutes, and counting.
I don't know the exact hour count for any others, but Super Smash Bros. Melee and Animal Crossing: Wild World are both games I've probably spent a week or more playing.
GTA San Andreas. Not sure how long I played this game, but I think I got close to 180 hours all together.
GTA III by a pretty wide margin. Second I don't know, probably one of the old Pokemon games.
Lost Planet 2. So many nom de guerres to go back for. I think I've spent maybe 120+ hours playing and I'm still no where close to getting them all.
GTA 4 all stories
It's probably between Pokemon Red, Blue, and Yellow versions when I was a kid and Fallout 3.
Oh, and Original Super Smash Bros. which I played after school almost every day for years.
FEAR THE MONARCH!
I'm going to cheat and say a series. The Quest for Glory series for the PC easily consumed 500+ hours playing and replaying them over the years. Classic point and click gaming at its finest.
Oblivion, probably 170+ hours over multiple playthroughs. After that probably Fallout 3 and, oddly, Lost Odyssey, simply because I've played it like 3 times.
.Hack G.U Redemption. I had well over 100 hours in that game. The longest yet.
The DS Pokemon games. Diamond I have around 60 hours in..but I'd say combined with Platinum, SoulSilver, and Black, I have well over 300 hours invested in those games.