Super Mario World is definitely my favorite of the 2-D Mario platformers. The primary reason is its secrets. No other Mario game has had to many secret exits and such a non-linear world. Sure other games have lots of items, but it's not about items, it's about finding new places to explore and not being forced down a one-way street. No other 2-D Mario game (except for SML2, which was somewhat based on SMW) has contained such a magnitude of these elements, and thus none have matched SMW's brilliance. Additionally, SMW was the first to introduce elements that can actually change other levels, such as the Switch Blocks, and the only one to include an evolution of the overworld landscape itself. SMW was a game that had a perfect balance of exploration while still retaining the classic Mario platforming feeling where you knew you were going for the goal posts at the end. SMW was a magical introduction to the SNES just as SMB was to the NES.
The debate between SMB3 and SMW is endless, but for me SMW is the epitome of the series. Sure, the suits were cool in SMB3, but SMW distilled Mario's powers down to the essentials while giving him new physical tricks like the upward throw, and of course, the addition of Yoshi was a game-changer. SMW's base was SMB3, as proven by early versions of the game, but the expansive and colorful environments really made a difference. The sheer number of ROM hacks for the game show just how great of a game engine the game had. On the negative side, SMW did seem to lose some environment diversity, and it was the first Mario game to include several music tracks based on the same tune, though the music itself was good and the presence of Yoshi adding a drum track was genius.
I have a lot of fond memories of SMW, even before the game came out. When I was younger, I always got consoles several years after their release. I didn't get a Super Nintendo until about 1995; my parents bought me a TurboGrafx-16 for Christmas because it was cheaper. But I remember seeing Super Mario World featured on GamePro back when they had a Saturday morning TV show. They showed how to get to Special World. In 1994, I was able to play my uncle's SNES when we were visiting my grandfather. I remembered the steps to finding Special World, managed to complete them and spent that vacation navigating my way through its brutal courses.
Nintendo Power's SMW Player's Guide, Mario Mania, was probably their greatest effort. Not only did it lay out all 74 levels and 96 exits in the game, it also gave a history of Mario that was relatively unknown at that time, and even included statistics like number of blocks that Mario can jump in each game, depending on whether he's running, etc. It was that kind of detail that really showed dedication, a kind that is hard to find now.