From what I've heard, it is easily Willie Mosconi. And I say that because there was a story I read on these forums a while back which had to do with one of Mosconi's exhibitions. After everything was done and all the people applauded, there was one lady who did not think much of what she had just seen. When asked why she didn't find it impressive her answer was something along the lines of, "He never had one hard shot. How was this at all difficult?"
In person, it would have to be Karl Sloezen, who I got a lesson from. And I'm not saying it because he gave me that lesson, but I've never seen someone run rack after rack of straight pool while holding conversation as fluid as he did and explaining what was happening. And he was doing this after showing up, not warming up and on the tightest table in the hall.
From watching videos of guys in their primes? Either Buddy Hall or Rodney Morris. I like Rafael Martinez and thanks to AccuStats I've been able to watch more of him, but he is more of an entertainer than someone who I think made it look easy. Making it look easy to me means you just go up there and shoot. Rafael obviously had to do some thinking before attempting those insane shots.