Bad: Peter Lorre as Mr. Moto in Mr. Moto's Last Warning. As a film, the best of the three (ok, the only one I watched to the end) but still, it's... Peter Lorre being Peter Lorre with glasses and partially blacked out teeth. Possibly worse was George Sanders pretending to be French, or maybe Italian... I'm still not sure which.
Worse: Boris Karloff as Mr. Wong in Mr. Wong: Detective. It's... Boris Karloff.
Worst: Bela Lugosi as the bad guy in The Mysterious Mr. Wong. It's... Bela Lugosi. Y'know... Hungarian is barely related to other European languages, much less any Asian ones. I dare you to summon the suspension of disbelief required to make this less than hideously jarring.
(Not in the running, but still the all-time worst: Mickey Rooney in Breakfast at Tiffany's.)
In my mind (if nowhere else) there's some sort of sliding scale of year vs. caricature that regulates my opinion of these things. Peter Sellers as
Milo Perrier: What do you make of all of this, Wang?
Sidney Wang: Is confusing.
Lionel Twain: [from moose head] IT! IT is confusing! Say your goddamn pronouns!
Sidney Wang: Oh, there, voice come from cow on wall...
Lionel Twain: Moose, moose you imbecile!