wow, that's a tough one actually. I'm just gonna rattle off a few in no particular order - probably not my top 5, but some games I've been addicted to over the years.

Gunship 2000 - loved this game to no end. Used to play all day every day just trying to make that next rank. One of the first games I saw where you were given a rank as well - and if your helo was destroyed, a very good chance that you were dead for keeps, and had to start over in the career.

Marble Madness - Liked this one mostly cause I was good at it :) to this day, none of my friends will even play me. Really fun, and always had your fingers thinking.

Contra - needs no introduction, awesome co-op game, and I'll keep playing it just to see if I can get through it without needing the up up down down left right left right b a start........or select start for 2 player :)

SWG - my first MMO, and I'll be honest, I did quit for a while after the you know what came out, but I re-activated a couple months ago and am playing pretty much every day. Love the movies, love the game.

Super Mario Bros 3 - my favorite of the long running series. Not much info needed as I'm sure we've all played it once or twice.

mine are

1. halo 3 (thats a given) - xbox 360

2. legend of zelda ocarina of time - n64

3. Oblivion - xbox 360

4. star wars knights of the old republic - xbox

5. ninja gaiden - xbox

World Of Warcraft :)