I'm replaying SMB3 for probably the 100th time. And it still is fun for me. After playing it since 1991, I still love the colors chosen for the stages, the graphics, the music (despite the low amount of tracks), the sense of discovering its hundreds of secrets hidden in each and every stage. This is nothing new, as lots of people think the same, but it's awesome this game still feels like one of the best platformers ever. When the only flaws you can find in a game is its lack of save feature and the overworld map pipes being not that useful, you know you are in front a very special game, that comes maybe once in a lifetime, a work where its creators were truly inspired and created a masterpice.
Lots and lots of games have been released since then, and some are between my favourites. But SMB3 is (along with Sonic 2 ) one of the only games I still play through it completely twice a year and then some stages at least each month.