Super Mario RPG: Legend of the Seven Stars (SNES, 1996)

Of all the genres Mario touched upon, the RPG was probably the last thing anyone expected.  The Mario games are all about action, not about careful strategy and turn-based battles.  At this point, Mario had done racing games, puzzle games, and had yet to star in a plethora of sports games.  Nintendo wanted to put Mario in an RPG, so they gave the full creative license to Square, the geniuses behind Final Fantasy and Chrono Trigger, to make it.  What they came up with was a brilliant game that both helped popularize RPGs in the West and stay true to the Mario canon.
Square had tried earlier to make a simplified RPG for American beginners to the genre, Final Fantasy: The Mystic Quest for the SNES, and it had been a colossal failure.  The game pushed insulting when it came to how easy and dumbed-down it was.  American gamers rejected it outright.  Super Mario RPG: Legend of the Seven Stars (1996) is the right way to make a simplified RPG, that doesn't underestimate its audience or hold their hands, and offers depth and fun at a level low enough for new-comers to enjoy.
Super Mario RPG starts off like a typical Mario game:  Bowser kidnaps Princess, Mario must go to Bowser's castle to save her.  However, during Mario's rescue mission, a giant sword descends from the sky and plants itself into the castle, destroying the bridge to star road (the place where people's wishes are granted).  The sword has come from another dimension and belongs to the Smithy clan, who wish to take control of Mario's world as well as theirs.  The only way to fix star road, where the trans-dimensional portal is located, is to collect the seven legendary stars.  Only then, can Mario fight off the invaders.
This certainly sounds like a serious story, darker than the typical Mario fare, and it is, but the game takes a light-hearted approach to it.  This game's humor would become the standard for future Mario RPGs (such as Paper Mario and Mario & Luigi).  Consider the scene where Mario explains to the Toadstool king what happened at Bowser's castle.  In pantomime, Mario acts out the events with a slapstick quality that evokes the likes of Charlie Chaplin or Buster Keaton.  He walks forward, representing his approach to the castle, then looks up and makes a bewildered expression, then jumps backward frantically.  This is only one scene among many that inspires genuine laughter.
The game's RPG elements also mesh well with the Mario aesthetic and universe.  Mario roams an isometric, pseudo-3D landscape, where enemies can be seen in plain sight.  If Mario runs into one of these enemies, a battle is initiated.  You may have three party members in a battle, and they include the cloud-person Mallow, the living doll Geno, the Princess, and even the King of the Koopas himself joins your party.  Each player and foe takes their turn attacking, and you may make a standard attack, or a special one, which costs TP from a shared pool among characters.  What makes the battle system in Super Mario RPG special though is the battle command system.  When doing a normal attack, press the attack button at the right moment and you can deal extra damage.  Special attacks require their own intuitive button imputs.  When being attacked, press the button at a precise moment and you can mitigate the damage you receive.
Audiences were originally alienated by this game (especially younger players).  The combination of Mario platforming and RPGs was confusing at first, but players eventually warmed up to it.  The game offers some of the best gameplay and visuals on the SNES.  All of the backgrounds and character sprites (and animations) are pre-rendered by a computer, so everything is detailed, bright, and colorful.  The music is lively and eclectic.  Beside the inevitable remixes of the classic Mario themes, there is a lush soundtrack that pushes the SNES's sound to the limit.  This game would be the genesis of the Mario RPG, which would come far in between, but always something special.