The spelling I'm most familiar with is Thalia.
I've always heard there was this difference:
Tahlia/Talia is a Hebrew name with a hard "T" sound in the beginning.
Thalia is Greek and the "TH" is like the 'TH' in "the"
Interesting. I do know a Jewish Talya, but the Thalias I know use the hard T sound. There is some variation as to whether they say TAL-ee-uh or tuh-LEE-uh, though.
FWIW, I think it's pretty in any spelling. I'd rank them Thalia, then Talia, then Tahlia. I don't care for Talya, even though that's legit, too.