I find MMORPGs to be cheating, considering you can effectively talk to anyone in the world through such channels and gather information on how things are going, which kind of defeats the point of the Desert Island question. So I'm skipping online games. Also, given that I may as well enjoy myself with at least one new game, I'm not going to restrict myself to games that I've already played. Long games will probably help pass the time.

Final Fantasy VII - Favourite game, good replay value.
Warcraft III + Expansions - Good replay value, particularly if you have bucketloads of maps.
Pacman - Excellent replay value!
Mass Effect - I hear this has something like 70 hours of gameplay + good replay value.
Civilization IV + Expansions - Fantastic replay value with randomly generated maps and the increased difficulties should also give me something to go for in the long run.

Gran Turismo V unlucky to miss out. I'm not a massive racing fan but it would provide good change from the other games and allow me to relax while listening to decent music. In fact, now that I think about it, I might take it in place of Pacman for the addition of music value. We all need music!