I think the only way to judge this is by looking at each comedian at their best (Best standup comics of all time).
You age at the time you saw these people has a lot to do with it. Mort Sahl and Lenny Bruce were before my time. I remember some comedians but there were over my head as a kid: Bob Newhart, Alan King, Joey Bishop.
I never "got" Richard Pryor but he obviously suffered from being compared to Bill Cosby who as a kid I thought was just hilarious. As a kid I also like Alan Sherman (not on there as far as I saw) a lot.
Johnny Carson was very popular - biggest sellout in Vegas in his day (back when Vegas had BIG stars) but was he that funny?
The ones I have seen do solo standup shows (live or on TV) that "killed" include (no particular order) Robin Williams, Steve Martin, Chris Rock, Eddie Murphy, Joan Rivers (at her peak).
I remember Freddie Prinze was very funny at his time (died young, too bad). I saw David Brenner in Vegas and he was way funnier than I expected.
I liked George Carlin, but not when he was younger (hippy-dippy weatherman?) - but his last stuff was really good.
I like seeing Andy Kaufman a lot more now than I did when he was "famous." Knowing his real life makes what he did much funnier.
New comedians - none of them. There is actually a Sinbad HBO show where he is really funny - better than Jerry Seinfeld (never found his act even remotely funny).
Dennis Miller is higher than Bill Maher - but they are both more commentators now than comedians.
George Burns and Gracy Allen should have been higher - they were classic.
I dont care for Ray Romano, Wanda Sykes, Jonathan Winters, Albert Brooks (at all),