Bowser

King Bowser (sometimes known as King Koopa) is a major character and the main antagonist of the Mario franchise. He's the Koopa Troop, an antagonistic organization of turtle-like creatures, and has been the archenemy of the Mario Bros. since his debut in the first Super Mario Bros, repeatedly kidnapping Princess Peach (or trying to do so) with the ultimate goal of defeating Mario and taking over the Mushroom Kingdom.

Why He's a Grade-A, 100% Prime-Cut Final Boss

 * 1) When Bowser was created, he was a typical villain without much personality beyond simply wanting to take over the Mushroom Kingdom. Later games started showing various different aspects of Bowser's character, depending on the nature of the plot.
 * 2) * While his platformer role continues to be that of a simple antagonist, his Mario Party antics typically depict him being more angry, childish, and mischievous than dangerous, and his portrayals in the RPGs range from a threatening, yet humorous villain, to a playable anti-hero.
 * 3) * Even when Bowser is acting as a protagonist in a game, however, he continues to insist he is an evil character, and his reasons for helping the protagonists are typically his own selfishness. In some games, he even breaks the fourth wall to do so.
 * 4) He's more than a generic princess-napping dragon, he's the king, and he'll make sure you know it. The Koopa King has a sense of style and showmanship when it comes to his dastardly deeds, after all, he is the King of Awesomeness.
 * 5) *The biggest examples of this would be in Super Mario 3D World and Super Mario Odyssey, the former having Bowser ride in a gold-plated muscle car with an entire world themed after a lit-up carnival, and the latter having Bowser dressed in dapper wedding attire for his big day.
 * 6) *Even in the more standard games, Bowser shows off his ego by having his castles designed in his likeness, many of them having his head as the entrance and his spikey shell as the roof.
 * 7) While not exactly a genius, he's a cunning and clever while still remaining an arrogant brute, take for example, Mario & Luigi: Bowser's Inside Story, where he's easily tricked by Fawful multiple times, but also points out strategy and tactics in battles where there's a gimmick and taking it to his advantage. In Mario & Luigi: Dream Team, Bowser plays dumb for most of the game and betrays Antasma once everything falls into place for him, also getting to be the all-powerful final boss of the game.
 * 8) He might act tough, but Bowser does have a soft side underneath that intimidating shell (pun entirely intended). He's a single father who tries his best raising Bowser Jr., and he treats his troops with respect.
 * 9) He has a perfect villainous design for his species, combining standard Koopa elements with those of a fire-breathing dragon.
 * 10) Scott Burns and Kenny James are awesome at making him sound like the goofy evil beast he is, and Jack Black is sure to give an awesome performance in the upcoming movie.

The Only Bad Quality

 * 1) His portrayal in the Super Smash Bros. series lacks the humanity that makes him so endearing, being nothing more than a fearsome beast. He doesn't even have his voice from the mainline series, instead only being generic monster noises. What makes this especially baffling, is that Bowser Jr. and the Koopalings are all represented quite faithfully compared to Bowser himself.
 * 2) *His appearance in the first New Super Mario Bros. also lacks his personality and voice, but this is thankfully not an issue in it's sequels.