It will depend of what you are really expecting in regard to the weather. If you want to feel a uncomfortable and disgusting heat, then come from middle december to march.
On the other hand, abril through october, for me, is the best period of climate we have here. I suppose you might be european, so do bear in mind that what we call "winter" in Rio, is something about 18-22 C. Ridiculous, isn't it?
Anyway, have fun whenever you decide to visit my city. Regards almost 8 years ago