I'll confess myself as biased on the name
Kathleen. I have an absolutely horrible connection to a
Kathleen. She was really cruel to me and one of my sisters. The name
Kathleen is a great classic, but I would use the spelling
Cathleen, just based on that association. However, I prefer
Catherine over
Cathleen.
Kathlyn is okay, but I can only see it as a modern variation of
Caitlin. I'd never heard of it until now, though.