Music makes the game. A well-composed score written to accompany narrative and gameplay is vital to preserving the game's overall theme. The right song at the right moment can establish mood in a new setting, charge an encounter with emotion, and make the player forget where he or she is altogether. If a video game is a recipe, then the game's score is the seasoning. You may be able to enjoy a game without good music, but you can never savor it to its greatest potential.
The following slideshow is a collection of video games made infinitely greater by the soundtracks that accompany them. For each of these games, I've done my best to select one song to represent that game's personality. Keep in mind that this is not a ranked list, and that given the opportunity to create one, your list might be different than mine.