1. MGS4 (It's MGS. I could play it forever)
2. Little Big Planet (all the time in the world to make my own levels!)
3. Killzone 2 (the best FPS with bots I could think of)
4. The Orange Box (4 games in one seeing as how the fifth is online. But 4 of the most top notch games ever made? OK!)
5. Oblivion GOTY edition (I like Fallout 3 more, but I felt like there is more to do in Oblivion and it has more room for multiple playthroughs due to classes like mage, knight, theif etc. which all play differently.)

sims: cast away duhh

Uh

Damn

I guess I'd want a variety of games to play, at least one of them that I love so much that I would never stop coming back to it, at least one that I could improve at forever....

1. Harvest Moon 64/Rune Factory Frontier (do not ask me to choose, here, either would work)
2. God Hand (I can improve at this endlessly)
3. The best Fire Emblem (either FE7 or Radiant Dawn, maybe FE7 for the greater amount of potential party variety)
4. Civ IV (by the time I'm good at it I'll have already put in like a month of playtime without breaks)
5. Steel Battalion

Yeah, that'll do

Racing - Gran Turismo 4
Action RPG - Baldur´s Gate Dark Alliance 2 (PS2)
Sport - PES
FPS - Half Life 2.
Sandbox - GTA San Andreas

wait... where would you plug in your stuff? a tropic tree?

My 5:
Animal Crossing: City Folk
Excitebots: Trick Racing
Digimon Rumble Arena 2
Punch-out! (Wii Version)
Band Hero

all have infinte replay value for me, I'd be set for a while

Demons Souls is all I would need, because it's unrelenting difficulty would make the dessert island seem like a cakewalk. 

I'd trade every game for a hot, willing girl with a great personality to be stranded with me

The Sims 2: Castaway

Adventure Island

Gilligan's Isle

Swiss Family Robinson

Idols Galore!

This one