Whether you choose to look at the Ewan McGregor version or the Alec Guinness one, there's no question that Obi-Wan Kenobi is a Jedi Master of the very highest order. He was extremely gifted in combat, defeating two Sith Lords, but even more than that it was his attunement to the Force, the way he followed Jedi principles, and his wisdom, that gives him so much power. After all, he trained both Anakin and Luke and, even if they surpassed him, it still says a lot for his own ability. When we meet him in A New Hope he may not be the Jedi he once was, but he still displays his command of the Force, his fighting ability, and his knowledge and selflessness. After all, in allowing Vader to strike him down, he becomes more powerful than you could ever imagine.