"I've always said two things: He's the funniest person I was ever around; and what we see is a manifestation of deep insecurity."
Their late night rivalry was a thing of TV legend.
"In Jay's case, it was not so much joke writing as it was... his attitude was hilarious," he said. "So whether he had a joke or not, it was funny. And people loved him."
He revealed their rivalry began way before they became warring kings of late night TV ratings, back in their comedy circuit days, when he admitted gathering at the back with other comics to heckle Leno just "because he was fun to heckle".
"He didn't like to be heckled, because he was working on stuff, and we were just getting in his way," he laughed. "It was the best."
Leno got his revenge though; when he got wind of a stand-up gig Letterman was bombing at in Denver, he called up his wife to insist he be the one to collect her husband at the airport.
"Jay Leno hears that I'm bombing, and he loves this,' he recalled. "He calls my wife and says (slipping into an extremely high-pitched falsetto Jay Leno impression) "I'm gonna pick Dave up from the airport when he comes home."
"So I get to the airport and there's Jay Leno and he's (goes up a few octaves again) "Hi I understand you had some trouble Mr Letterman?" he chuckled. "I can't even get home without the ignominy of having bombed in Denver."
He admitted Leno, like Jerry Seinfeld, had the "stand-up gene", and could make a career of comedy on the road... something he himself did not have.
As legend goes, Letterman was assumed by most to be the heir to Johnny Carson's "Tonight Show" — including Carson himself — and was hoping to move up to the earlier slot from his "Late Night" hosting gig. But when Carson eventually retired after three decades in 1992, it was Leno who took over the show.
As a result Letterman defected from NBC to rival CBS, and the "Late Show" was born, broadcasting at the exact same time, which continues to be a direct ratings rival to this day.
When host Maron pointed out that Leno, to his credit, was a guest on his show even though he knew Maron was in "Letterman's camp", Letterman reiterated his praises, but added Leno was deeply insecure.
"I've always said two things: He's the funniest person I was ever around; and what we see is a manifestation of deep insecurity. But as a stand-up, was there anybody [as good?]"
He added: "Nothing happened on which you could indict. And he was good. Really good."
When asked if he could envisage a future in which they worked together, he said "no, not really," but added: "We were friendly. I liked him."
Letterman spoke of many other comics, including Robin Williams, who he said almost made him walk away from comedy after seeing him perform.
"You know in the NFL there would be a quarterback who doesn't stand in the pocket, and just throws passes... there's a quarterback who will run sideline to sideline to the opposite end zone, and then throw an 80 yard touchdown pass, and everyone says 'wow, that's the future of being an NFL quarterback.' We thought that about Robin," he said.
"George [Miller] and I used to watch him work, and we just both thought together: "We're screwed. I'll just go back to Indiana. I can't do that."
"He was so effortlessly effective, the place would explode, and you couldn't follow him. Could you get an audience to explode and blow the roof off the place?"
Another impossible comedian to follow was Richard Pryor, who was like "oh my God Jesus Christ has come to do a set."
"Richard said goodnight, and so did the audience," Letterman laughed, recalling the only thing comics who played after him could joke about was how pointless the exercise was.
Letterman also admitted still being haunted by his treatment of the late Bill Hicks, who was "sloppily" cut from his show after doing a set that was deemed too offensive at the time.
"That was a mistake," he said, noting Hicks died of cancer afterwards before he had a chance to make amends.
"It was discourteous, it was impolite, it was inconsiderate on my part. It should not have happened. I have no great defense... I had really been a fool," He lamented, admitting that the set wasn't even that offensive in retrospect. "'Really? Really Dave? You did this to this kid?' One of the many mistakes I made."