Probably the hardest game in existence, and it's mostly the original Super Mario Bros. with levels 20x harder
This is regarded by many, many people as one of (if not the) most difficult game ever created. It really is just SMB with new, much harder levels. The good part about that is that SMB has a near-perfect control scheme and awesome gameplay, which this game perfectly matches. Minor changes are the sound quality, which is slightly better, the ground is now dirt and not blocks, and the backgrounds now have some detail, rather than just a colour.
In the original SMB, I could barely make it to World 8-3. On here, it took me forever just to beat World 3. Honestly. This game is so ridiculously hard that you are a god if you beat it, and that's not even mentioning the 5 secret worlds added to the end. Some gameplay changes from SMB are the invisible gust winds than either propel you forward while jumping or push you back, often killing you in pits of death.
Other than that, it could be though of as an add-on of the original SMB. An extremely difficult add-on. Despite it's extreme difficulty, this game is every bit as fun and addicting as SMB. Though the frustration levels might reach somewhere over 9000...