I can recall several iconic Martin appearances on talk shows. There was his Johnny Carson-requested performance of The Great Flydini (his only performance of that act on camera) for one of Carson's final Tonight Shows. There was the Late Night with David Letterman appearance in which Martin showed up with a doctor and begged out of talking to Letterman, feeling sick. Dave offers Steve the chance to lay down and rest in Dave's office, but during periodic check-ins throughout the show we see Martin first planning and then hosting a giant, raucous part in Dave's office.
There are dozens more than come to mind. Of course, humor is a very personal thing, and your mileage may vary. But for my money, Steve Martin's funniest-ever talk show appearance happened in 1980 on Letterman's short-lived morning show. Having worked very hard the night before (so the bit went), Martin is wheeled out in bed. He's asleep, yet clutching a six pack of beer. There is a small, portable TV at the foot of the bed.
Here is Part 1 of the appearance (forgive the very low quality, but clips of Letterman's morning show are difficult to come by):
The phony commercial, from a TV special Martin is plugging, is hilarious: It's for "Honeymoon Butter," which is, literally, made with love.
If anything, the laughs get bigger (for me, anyway) in Part 2, once Martin, still clad in pajamas, leaves the bed for the chair next to Letterman:
It's easy to see why Letterman became such a late-night legend, why Steve Martin was always one of his favorite guests ... but also why Letterman's morning show was canceled after just a few months. Imagine the people watching daytime TV in 1980 — accustomed to only seeing game shows, soap operas and talk shows like Phil Donahue and Gary Collins — stumbling on this.