To support my reasons I have included numerous examples from TCW episodes; however, since I know more people know him from the show than the short books and comics he's been in, I'll use TCW for most of my examples. Note: Just to clarify beforehand, I didn't arrange the reasons in any real order, so it's not like it'll come down to my "top reason."
Hopefully, by the time you have finished reading it, you will better understand the underrated and overlooked character of Cad Bane, as well as why he stands as my favorite!
Reason #1: Character Inspirations
The first reason I love Cad Bane is because of the inspirations that formed him. When it comes to creating characters, there are usually at least two major sources from which to inspire the character - their appearance and their voice. Both of these eventually contribute to the overall personality of the character. The inspiration for Bane's initial design - which were taken from original sketches from the first Star Wars movie - was none other than old-time actor Lee Van Cleef (see right). Lee Van Cleef played a lot of different characters in his lifetime but he is most famous for his westerns. If you have seen or heard of the "The Good, the Bad, and the Ugly", Lee Van Cleef played Angel Eyes, "the Bad". As Angel Eyes he had such a cold, cruel, defining presence on-screen, which brought me into an Old West obsession in 4th and 5th grade. To be honest he was one of the first on my list of "imaginary husbands". Not only was he a talented actor but he has become a legendary icon over the years. He did a fantastic job playing any sort of dark character, especially his villains, and the fact that he was used as the inspiration of a Star Wars villain is fantastic in of itself. If you haven't seen any Lee Van Cleef films, I highly recommend "For A Few Dollars More", "High Noon" , "Commandos", "Diamante Lobo", "Sabata", "The Grand Duel", and of course "The Good, The Bad, and the Ugly"!
I already love these two actors and their works to begin with, so learning they inspired my favorite character was a huge bonus! At the same time, they both embellish the two parts that I think blend together well to form the character of Bane. On one side, Cad Bane is the badass gunslinger reminiscent of the Old West, not afraid to fight Jedi and quick with a blaster. And on the other, he is manipulative and capable of getting out of sticky situations without firing a single shot (two aspects of Bane I'll talk more about later). Putting these two together, from two very different sources, creates a unique character who embodies the awesomeness and the darkness from both. Thus this forms a new presence on screen who is talented with both gun and tongue, the lovechild of a 1960's spaghetti western and a 1930's film noir. In a word, he is the combination of the toughness of Lee Van Cleef and the slyness of Peter Lorre.
Reason #2: Role in the Saga
Herein, between these two Fett's legacies, lay a dark, unoccupied gap. The Fett saga is put on pause during the Clone Wars and the Rise of the Empire era. We know Boba would eventually reclaim his father's title as number one, but that would not happen for at least twenty years or so.
In the period following Jango's death and prior to Boba's rise, there needed to be someone else who took a turn at the top of the pile. Someone who would take advantage of Jango's death to become the most notorious bounty hunter of his time. Someone who could exist as a strong motivator for Boba to carry on his father's legacy and bring the Fett's back to their reigning status again. And of course while all this is happening, a galactic war has begun which has forced the peaceful Jedi to become warriors again. War on such a grand scale invites all kinds of treachery, violence, and disarray, things which folks loyal only to the highest bidder can, and will, take advantage of. Wartime is when scum and villainy such as bounty hunters are in their prime and find the most work. Obviously the Clone Wars was such a time for many new mercenaries to come on the scene, and someone had to be the best of them all.
Needless to say, Cad Bane filled this role perfectly. While we have yet to see the rest of Bane's story play out in the saga - and who knows where it will be taken - he has so far obtained an ideal part in the story of Star Wars. The story, that is, of the criminal underworld. The Clone Wars era needed a new bounty hunter to come up on top, a character who reigned between Jango and Boba Fett. It compliments not only the Rise of the Empire era, but fleshes out the characters well, especially that of Boba. Argue against me all you want, but there is no doubt that the Fett's are legends in the Star Wars franchise, so it was essential for a character outside that name to set the stage for later events. In short, Cad Bane acts as both a worthy successor to Jango and an ideal predecessor to Boba's eventual rise to glory.