I think it is hard to argue that anyone has been a better champion than Hughes or Anderson. Very dominant champions for long periods of time. I think GSP is another guy who has been one of the greatest champions we've ever seen.
If anybody, GSP has had one gimme in his career (which he would lose of course) with the UFC. His debut was against Karo and he fought tough fights all the way to where he is now. Even Anderson and Hughes can't say that.
Randy is a great fighter, but I don't think he is a great champion. He usually wins the belt and loses it soon after.
I am looking at defenses and who they beat to hold onto the title. Like Tito for example, he had many defenses, but he was not beating the guys that are around today. Same can be said for Frank Shamrock.
Hughes, Silva, and even GSP, have (were in Hughes case) been fighting the top people in their divisions.
If I had to pick one pick, it would be Anderson Silva.