I think the top 10 games that have eaten up my time are:
NES Mega Man 3 - Played it over and over doing my best to avoid taking damage and finish the game as quickly as possible.
SNES Final Fantasy III(US)/VI(JPN) - Multiple playthroughs trying new different character each time, making sure each character got to lv99 & learned every spell.
SNES Chrono Trigger - Multiple endings = multiple playthroughs. Trying out new teams. Gotta love those double & triple techs.

Starcraft/Starcraft: Brood War - Story mode on each took a good bit of time, but the bread and butter of the game was competing against others on battlenet
PS1 Final Fantasy VII - Such a phenominal game. Spent a bit too much time getting all the playable characters to lv99 & getting multiple copies of the Knights of the Round materia.

Disgaea - Item world. That is all.

Super Smash Bros. Melee - An interesting fighting game with an even more interesting cast of characters. Spent a lot of time sharpening my technique with Jigglypuff, Peach, Samus, Mario, Marth & Sheik.

Fallout 3 - Huge world, multiple playthroughs (primarily for weapon skill preferences) and karma.
ARCADE Marvel vs. Capcom 2 - There were some balancing issues, but it was always interesting learning a new character or trying out a new team.
ARCADE Capcom vs. SNK 2 - A fantastically balanced fighting game with an incredable roster. It took me a number of quarters to get comfortable with the K groove.