I feel that next to Anakin, Obi-Wan's story from TPM to ANH (and even beyond) is such an amazing journey. Obi-Wan truly is a sad character when it boils down to it. He lost Qui-Gon, he failed to complete Anakin's training and lost him to the darkside, and he witness the deaths of thousands of his Jedi allies die one by one before him as the galaxy crumbles. Like Yoda, he spends twenty years by himself on Tatooine. But besides such a compelling story, I think what makes Obi-Wan such a fantastic character is that he gets into the best adventures. He faces different sith lords in battle, fights Jango Fett (probally the most feared bounty hunter in the galaxy by the time of AOTC), and so on. And let's not forget that Obi-Wan sacrificed his life for the good of the galaxy.