ARTISTS' BRUSHES

I am thinking to renew my collection of brushes and I had ordered catalog from Rosemery&Co. The diversity of brushes give me a dilemma which one to choose. For example, they have a good collection of Nylon brushes, then bristles, or combination Nylon plus Hog Bristle etc. Mainly I work with acrylics and that narrow my choices, but regardless of that I still have a lot to decide.
I would love to hear from you which one is your favorite brushes?
What sizes do you use?
Do you prefer rounds or flats or some special effects brushes?