The 74-year-old said the amount of "brilliant new stuff" today is "small" and he has seen most of it before.
Speaking at the Cheltenham Literature Festival, Cleese added: "When you've been doing comedy for as long as me, you really know most of the jokes."
But he said Bill Bailey and Eddie Izzard had grabbed his attention.
"This lights you up because these guys have done something new and brilliant and that's marvellous."
Cleese told his audience that "great old comedy" from Keaton and the Marx Brothers was difficult for new comics to live up to.
"You don't expect anything great - you turn on, you watch it for a few minutes and you think, 'It's fine, I've seen something like this before and it doesn't excite me," he added.
But the comic actor, who co-created and starred in classic sitcom Fawlty Towers, said other areas of life still made him laugh - notably politicians.
"What was happening at the Scottish Referendum was absolutely hilarious," he said.
Cleese appeared on The Graham Norton Show last week and made headlines for insulting pop star Taylor Swift's cat as she sat beside him on the chat show sofa.
"That's the weirdest cat I've ever seen in my life," he said.
Cleese was in Cheltenham to promote his new memoir So, Anyway.
Earlier this year, he reunited with Monty Python for a string of live dates at London's O2 Arena, recently saying that he had failed to be excited by the performances.
Monty Python star John Cleese has said new television comedy pales into insignificance compared with the greats, such as Buster Keaton.