I’m a big Grant fan. (Arsenic and Old Lace!)
But Martin would get my vote. Got to see him on stage when I was in college. He was a hoot. And he shows up occasionally at festivals in the area where my mom lives (Ashville) to entertain and play his banjo. They love him.
I’d have to say Martin, as well. Just for the Rental Counter scene in “Planes, Trains and Automobiles.”
But if I had to say what was the best single comedic acting performance in a film, to me, hands down, it’s James Cagney in “One, Two, Three”.