The Errand Boy is one of the funniest most joyous films ever made and probably my favorite JL directed film.
But ultimately Tashlin's Cinderfella is maybe my favorite thing he's done, himself or otherwise.
Then generally:
The Nutty Professor
The Stooge
The Patsy
The Bellboy
Artists and Models
Rockabye Baby
Cracking Up
The Big Mouth
Who's Minding the Store?
The Disorderly Orderly
Three on a Couch
Hollywood or Bust
Which Way to the Front
Hardly Working
The Family Jewels
A few more Dean Martin and Lewis films that I'm not fond of, but really The Stooge and Artists and Models are both master class.