Replay value is "the entertainment value of playing a game more than once".

So what makes a game have replay value?
-unlocking secret features after the game ends (hard mode, extra missions, etc.)
-trying out a new "dimension" of the game (start with another main character, alternate endings, etc.)
-personal liking (the plot/music/gameplay suits your tastes)
-loyalty to a series (like replaying Kingdom Hearts while waiting for the next one to come out)
-striving for perfection (in games that make you collect items or that count your completion %)
-internet play (MMORPGs)
-multiplayer mode
-etc.

So here's my question to you:

Which game(s) has the most replay value? Give some explanation as to why, so that others might consider playing it!