Ms. Barch was the worst for me.

For me Tiffany's voice is the worst.

I found her FAR more tolerable than Stacey and Tiffanyso I'll give her that.
"I'll go,because I am Cinema!" - Ben (Man Bites Dog)

Tiffany's voice was very slow. I wasn't sure if this was some sort of joke about fashion-conscious girls.
I find Mrs. Barch's voice very annoying - actually worse than Brittany's.

No, that was Tiffany! Her voice got stupider and stupider! It went totally OTT after the third series. Yes Britney's did too, but Tiffany's was just stupid.

I work with a woman who constantly giggles - and it's high pitched and squeaky, too - she has some major issues.
I saw the best minds of my generation destroyed by madness, starving hysterical naked.

There was that episode "The Old and the Beautiful." Where Daria copied Brittney's speech as Brittney was coaching her including the squeaks. Apparently Brittney wasn't aware she squeaks.

I think that was the point lol. She had a fab "for a drawing" body and a cute apperance until she opened her mouth and that voice came out kind of wrecking the illusion of a hot chick.
Ever heard Paris Hilton talk normal? In the Simple Life she has a normal voice nice enough voice. However in interviews she makes her voice higher to sound "cutesy".
Like in the earlier episodes of Daria such as the first episode Brittney's voice is pretty mellow and sounds nice but as the episodes progress it gets the annoying high cutesy or in your words nails on chalk board lol.