Saki in Tatsunoko vs. Capcom. The fact that she is an utter banshee may or may not be a bad thing, considering it makes me fight her three times harder than I fight any other character, because I don't want to hear her screech any more syllables than she has to.
I mean, I hate everyone's voice in TvC; Frank West's, because it is so not his voice (but it is HILARIOUS! TO THE MOON!), everyone else's because they couldn't be bothered to get English voices (after all the trouble to get Ultimate All-Stars to be released in English-speaking territories, why didn't they go the whole nine yards? It bothers me). But Saki... is worse than knives on plates, nails on chalkboards, and knives and plates and nails and chalkboards in blenders.
Edit: There is one voice clip in TvC I like: when Casshan switches out with his partner character, it sounds like he's saying "Pound 'em."
"Pound 'em!"
"DON'T WORRY PAL!"