I don't remember GTO ever being dubbed!
Of course, I can say that the worst dub ever has to be IMHO "Battle of the Planets!"
Dishonerable mention:
Princess Mononoke
Tenchi series (Sorry Anubis)
Macross Plus. . . . need I say more.
Pioneer Akira
Sailor Moon
Project A-ko 2 through 4 and VS battles. . . . . . Same as macross plus.