Come now people…you're narrowing yourselves to English voice actors, it seems. The truth however, is that the WORST voice actor ever hails from the land of the rising sun. Ladies and gentlemen, let's have some applause here, because it takes flat-out TALENT to be as bad an actor as the one, the only.......Nozomu Sasaki!.
The man, the myth, the ruiner of characters who would otherwise be cool. His utterly inappropriate voice for Johan is the reason Monster needs a dub release as soon as humanly possible. His flat-beyond-flat acting is the real reason Yusuke was considered "dead" in Yu Yu Hakusho. His utter badness in his role as "Enishi" (or unreasonable facsimile thereof) augmented the levels of horror created by the Rurouni Kenshin: Reflections OVa to levels beyond human understanding!
In short, you'll not find anyone worse than him. Heck, even Moo might be an improvement.