Groucho Marx
George Gobel
Steve Allen
Alexander King
Oscar Levant
(I know those last two were not primarily comedians, but they were among the funniest, wittiest men ever.)
And lest we forget the ladies:
Lucille Ball (more a comic actress)
Totie Fields (a great stand-up who left us way too soon)
William Jennings Bryan
He used to do this bit about evolution that was just hilarious, just as funny today as it was back then!
I’m the first one to mention ** Benny Hill**?
Peter Cook without a doubt.
Graham Chapman (Where will the world be the day there are no Python boys left?)
All the lost Goons.
Douglas Adams, certainly.
But Cook is first.
Groucho Marx. Pure genius. His/Their work still stands seventy years later.
Mae West. I would love to hear her against say, “Bill I can’t keep my pants on” Clinton.
Rosalind Russell. Not a comedian in the truest sense, but excellent comic timing.
The Late Great Margeret Dumont.
Bill Hicks.
Another vote for Belushi, Radner, Madeline Kahn, Kovacs.
The Marx Bros and Bob Hope.
Phil Hartman, too.
Yes, and I sincerely hope you’re the last, too. 
What’s funny about sped up pictures of girls in bikinis?
The only possible answer to this thread has already been given. I discovered him way after his death, and I even miss him. And MAN, would he have had a field trip with the current US administration.
Michael O’Donoghue (More of a writer than a performer, but had plenty of creds on both counts). He and Bill Hicks were better-known to other comedians than to the general public, the way Alex Chilton and Tom Rush are better-known to serious musicians than to FM radio listeners, but their influence on their peers is incalculable.
To those of you who mentioned Bill Hicks…
Thank you. That man had gone undiscovered to myself, and he is definitely an amazing talent.
All of the good comedians today owe a debt of gratitude to all of those who ran before them redefining and pioneering various performance forms and forums. Lucille Ball for instance. Gilda, and John. Lenny? for sure. I miss the old Richard Pryor. I really miss Groucho. He was the best. Peter Sellers, the sad clown. Was going to say Andy Kauffman, but am not sure if he fits in this category. lol.
Lenny Bruce is kinda overrated. He’s known today as some sort of First Amendment martyr, but in his final days, his “act” consisted of him reading the transcripts from his trials and trying to intersperse it with some kind of commentary. That’s not groundbreaking, it’s masturbatory.
In his prime, Lenny was running second fiddle to Mort Sahl. We still have Sahl, and frankly, we’re no less lost for having him around. That goes double for Dick (“Bahamian Sawdust Diet Scam”) Gregory.