A classic that most gamers today have started from.
Mario World is especially good for those beginners, since it has the basic controls and then some that you can master after playing through the game a while. The first time you beat the game, you may be surprised if you've done so without going through most of the other levels, which may be a bit irritating, but are worth reaching, especially if you were able to do so without the guide book ('course, I was forced to use it...).
Even after you go through the various worlds and find every noock and cranny of the game, it's still fun to try and beat your old score by replaying the game in a different file. Sometimes you may just want to cruise through the levels with the amazing cape feather or any of the four Yoshis available for riding. Either way, it's a game you won't get tired of.
So in a nutshell, Super Mario World was one of the best SNES games out there and I, myself, am grateful that Nintendo made the right choice and put this as one of the first Mario games out for the GBA.