I agree. Vito added such a fantastic flair to every episode he was in. Even going so far as to own his own bar in Murder a Self-Portrait.
As you mentioned, the Undertaker scene is classic in Swan Song. Then as he is cast to play the down and out alcoholic bum in Negative Reaction. And turns in a stellar performance not just at the Mission when he sobers up but also at the junk yard where he meets Columbo for the 1st time. Great talent.
I agree with both of you. But let us not forget in a "Candidate for Crime," the scene in the tailor shop. That was very amusing. Also in "Any Old Port in the Storm," the scene in the restaurant with his fellow worker tasting the wine. I look forward to that scene every time I watch that episode.