The PC version is easily the best, so long as you've got a beefy-enough machine. Not only are the graphics improved over the console versions, but you can create a custom radio station from MP3s, and a handy replay feature lets you relive the last 30 seconds or so of stunts and destruction. You can also download user-created content that brings new vehicles, weapons and assorted phenomena to the game.
GTA: San Andreas is possibly the most ambitious game ever attempted, but that doesn't make it great. What does make it great is that it actually delivers on nearly everything it promises while still being fun to just pick up and play. Packed with enough stuff to keep even hardcore gamers busy for months, San Andreas is the ultimate time-waster for those old enough to play it.