As much as I like Columbo, though, Peter Falk's iconic role for me is -- and always will be -- the Grandfather who reads to Fred Savage in The Princess Bride. All he does is sit in a chair and gruffly read a book, but it's just so pitch-perfect a performance. The final line of The Princess Bride is, for my money, one of the best closing lines to a movie ever, and the movie gives that line to Peter Falk. The line? It's just three words, and Falk delivers it perfectly.
Here's a video I found, with Falk's entrance in the first scene and his exit in the last scene of The Princess Bride, complete with that oh, so perfect final line.
Thanks for the entertainment, Peter Falk!