can someone tell me where between margaret river and perth is the best wavesailing? How about bunburry? What about busselton? or is Perth (or just south of Perth) better?

It seems to me that the bay at Bunburry stops a lot of wind - is that true?

Nicolaas

You could try Yalingup, it's a bit more onshore and getting in and out not so easy, especially in big swell. If you drift into the shore break down wind, that's mast snapping material.
Busselton has no waves, and you're right about Bunbury, but it can have wind, just lighter than Perth.