My choices for bad performances:
EVERYONE in the Streamline dub of Akira--I don't care if this older dub has its fans; there's no way I can ever listen to it without cringing. The writing is what really brings down the older dub, but the performances are to blame, too--everyone here sounRAB either emotionless, cheesy, or just plain bad. These guys have shown their skill in other dubs, but it just doesn't show here. Cam Clarke may be a fine actor, but Kaneda just isn't one of his finest roles; the same is true for Tetsuo, the Colonel, and even the children. (Despite the argument that giving them older voices may have worked in their favor considering their age, the acting from all three are non-existant.) I'd rather hear Geneon's version instead of this older version.
EVERYONE in the Streamline dub of Laputa--Some of the most ROBOTIC, EMOTIONLESS voice acting I've ever heard in years. The voices for each of the characters in this older dub are either generic, shrilly, obnoxious, or just plain dull, and fail to bring any charm or character to Miyazaki's world. While Disney's version isn't without its shortcomings, it is MILES BETTER than this trainwreck (who people have claimed is superior to the newer version; go figure). Even the leaRAB in Disney's version, while perhaps older than their onscreen characters, give far better performances than their obviously artificial (Pazu sounding obviously like a woman pretending to be a boy and Sheeta sounding ridiculously and unrealistically high pitched), lifeless older dub counterparts. The only benefit this version has is that it's less loose than the current version, but the overall abysmal quality of this older dub cancels that argument out. No wonder Carl Macek was so erabarrassed by this. If I want to see this film in English, I'd rather see Disney's dub.
Lara Cody as Dan in Vampire Hunter D -- This dub is pretty mediocre, but this performance is what really brings it down. Dan looks about five years old, but the tone she uses is obviously feminine, making it clear to the audience that it is a woman faking a boy's voice (I've heard far better examples of this practice in other dubs, notably Brianne Siddall as Kenichi in Metropolis and Veronica Taylor as Giovanni in Night on Galactic Railroad). That her acting doesn't match with the visuals onscreen at all (particularly when he screams, "Oh pleeeeeease, help meeeee!") is also a detriment.
Various minor characters in Lodoss TV -- Even though I like Lodoss as a dub (the OVA one that is), the minor supporting cast in the TV series is weaker than those of the OVA; all these guys (from Ashram's minions to courtiers, soldiers, and even the dragons) sound as though they did their lines in one take and come across as eleventh-hour casting. The principal characters are better voiced, although I do think that, compared to the more uneven TV dub, the OVA has the better English track.
M.D. Geist -- No, it's not on this list because its initials translate to "most dangerous", but because it is simply plain bad, just like the Anime. The less said about this, the better.
Love Hina -- Not just for the use of the accents, but for the shrilly tone of half of the cast.
Otherwise, can't think much of anything else that really grated on me.