Cyia Battem (Ziyal the First) was good in the role, though she did make the character look to be in her teens--when you think of what they had in mind for her she did need to be a little older (the last thing you want is a creepy old man leching over a young girl...you know like they did with Neelix/Kes).
Tracy Middendorf (Ziyal the Second) was pretty awful in her one appearance. I don't know what it is but she just didn't do anything for the character.
Melanie Smith (Ziyal the Third) was my favourite actress in the role. She really gave Ziyal a strength and charm that made her very endearing, so that when she was killed off it was a shock to lose her.