Did you seriously think we were going to pick anyone else?
His tragic fall and ultimate redemption make him the most fascinating character in the entire saga. In fact, when one watches all six films, it becomes apparent that the series is more about Vader's redemption than Luke's ascension.
Vader's journey from asthmatic Sith Lord to smiling Force ghost was not one without some bloodshed. He could Force choke a high-ranking Imperial officer as easily as he could stand by and listen to the screams of Alderaan. Vader's heart was Hoth cold, something his son Luke soon discovered around the time daddy's lightsaber made Luke a lefty. As bad as Vader's Dark Side was, there proved to be some good in him, and the search for it took us all the way back to a young Anakin's "yippee!" beginnings.
Vader began life as an extremely promising young Jedi. Though many on the Council expressed reservations about his age and arrogance, Anakin quickly rose through the ranks and became the most powerful Jedi anyone had seen for centuries. Many believed Anakin would fulfill the Jedi prophecy and bring balance to the Force. He went on to win many great victories during the Clone Wars alongside his friend and master, Obi-Wan Kenobi.
We know how the story goes from there: Anakin's goes from pod racing to youngling slaying, thanks to his corrupt relationship with Supreme Chancellor Palpatine. The Emperor preyed on Anakin's tortured soul and used that to forge Lord Vader. Together they would wipe out the Jedi Order and rule the galaxy.
Luckily, we mentioned redemption. Anakin fathered twins before his downfall, and his son Luke fought valiantly to redeem his father, to bring him back from the brink. The Emperor rewarded Luke's efforts with a blast of Force Lightning. This was the final straw for the man that had once called himself Anakin. He grabbed Palpatine, absorbing the lightning for himself, and tossed the despicable creature down a chasm - Score one for the good guys. Vader was mortally wounded, but he had redeemed himself after decades of dark slavery to Palpatine. What's more, he had brought balance back to the Force in a long, roundabout sort of way.
His look, his sound, his aura, his tragic but redemptive tale? He truly defines the saga, and we love him for it.