Super Mario World is the best free game that I have ever played. The end.
No, I'm just kidding. It was the first game that I ever played for the Super Nintendo, and as such it set a pretty high bar on a powerful new console. One of the greatest testaments to this game is that it was free with the console (a novel concept). However, it featured incredible graphics and represented the powerhouse abilities of this revolutionary console. The game was so long that it kept you playing for hours upon hours, and still left you wanting more. The game over all was not too challenging to discourage the casual gamers, but not too simple to turn off the hardcore gamer either. The game blended inpecible gameplay, graphics, sound, and replayability. This game is a model for future games, and will always be the greatest Mario game of all time.