I really like
Katya. I have
Katia on my personal name list as that is the spelling I prefer but I can't help but think that some people may pronounce it KAY-tee-ah.
Katya would be better if you don't want people to pronounce the name wrong but in my opinion,
Katia looks nicer. I dislike
Katja. I don't like the way it looks and I think it will get mispronounced very often.