A few years back my wife bought a Super Nintendo on eBay. Now we have a 3 year old who likes to play Super Mario World but, quite frankly, sucks at it. I guess it's not his fault, he's only 3. The upshot here is that he asks to play Mario except what he means is that he wants to watch while my wife or I play. And he holds the extra controller and gets excited when each board is finished.
So I was playing Super Mario World yesterday and I'm happy to report that it is still freaking awesome. This is one of the best video games ever made. I'm amazed at how entertaining it still is. And it's actually still somewhat challenging. Not challenging in the sense that I'd ever lose. I played like 5 boards and collected 22 lives and I don't think I ever died once. But challenging in the sense that a lot of the boards have secret exits and you ahve to find the switch palaces of different colors.
In my opinion, the biggest leap forward in Mario gaming was from Super Mario Bros. 3 to Super Mario World. It is basically the same foundation of a game, except it's way better. It's more expansive, takes advantage of the greater capabilities of the Super Nintendo, and really doesn't have many, if any, flaws. Now I can't really speak for Mario games after this one. If you have a Nintendo 64 or if there are Mario games for the Wii, I don't know about any of that. But Super Mario World is still awesome.