15's are perfect, IMO; but I've seen rides on here with up
to 18's. They seem to always regret it, though

. If your
asking for a better offset/perfect offset for a particular size
of rims, I'd call tirerack or something, myself.
Your question's too vague for a decent answer. What are you looking at?
EDIT: You should post this in the
Performance-Tech/Rims&Tires section,
unless you find a "sticky" there that answers your question.