This question is the reason Smash Brothers was invented.
You are my new idol.
All joking aside, both franchises have left a tremendous impact on the industry. No, that's an understatement. Mario and Zelda MADE the gaming industry. How is it possible to compare two titans that are still standing strong after over 25 years of being buffeted by other franchises throughout history?
It's clearly that critics tend to side with Zelda a bit, as Zelda games generally have higher scores on both GameRankings and Metacritic. I've also been around the forums here for awhile and have seen other forums, and gamers usually come to the consensus that head-to-head, Mario is better than Zelda. However, when the title of "The Greatest Franchise" comes to mind, Mario is oftentimes omitted completely or is topped by Zelda and several other series.
Mario has been better represented this generation, as Super Mario Galaxy and its sequel have both been called the greatest platformers ever. In comparison, Twilight Princess and Skyward Sword, though spectacular games, haven't quite received praise on the same level in general. The tiebreaker is usually the fact that the Zelda series has Ocarina of Time under its belt and the fact that Mario is plagued with hundreds of crappy spin-offs that both publicize the mustached plumber and tarnish his name.
Zelda games are labors of love, and unlike in Mario games, the dungeons are strung together by a feasible story and an overworld. I prefer this over the level-to-level approach that Mario takes. IGN called Skyward Sword the greatest Wii game to date, and both SMG and SMG2 have received that honor. That is only one critic's opinion, but it's difficult to ignore that Zelda games like Ocarina of Time, Majora's Mask, Twilight Princess, Skyward Sword, the Wind Waker and A Link to the Past have been more warmly received than the 3D Mario games (overall). MM has been voted Game of the Decade, and OoT the greatest of all time. It's pretty clear that gamers believe Zelda is "the greatest" franchise.
In short, this:
Erebea said:
However, although Zelda has only around 15 games, it has sold over 52 million copies worldwide; while Mario has around 200 games and has sold 200 million copies. This basically says that every Mario games sells around a million copies each while each Zelda game has sold more than that. This makes what VanitasXII said about Mario always trumping Zelda sales false, as it is evidently the opposite in most cases. It may seem that Mario has way more sales than Zelda at first, but with a closer look you can discern that this is not necessarily true. In addition to this Mario games come and go at a rapid pace while Zelda games are carefully constructed, which puts Zelda ahead in the quality department as well as in dedication.
Zelda games are few, but their overall quality trumps Mario. Zelda wins by a hair for me.