My first thought was the same as Geeg, and I went to your profile to see what system you use to give you an informed answer, but there was no info there!
It would be great if you could fill your profile out as it really does help us.
Regarding your brush, Geeg is perfectly right that it depends on the system you use, the CND brushes are softer and perfect for pressing and smoothing out CND liquid & powder.
So if using CND get a CND brush a number 8 brush is best for less experienced tech's, and you can move on to the larger brushes once you have honed your skills.
In my experience, having used a few different brands, I have to say I always come back to my good old EzFlow Master Artist 508 brush. It is not as pretty as some brushes out there with its plain old wooden handle, but it does the job!!
It will flatten out for larger areas and for flush application in zone 3, and yet goes back into a point when required. Even though it is only a size 8 brush I can pick up as large a bead as I want with it.
I disagree (sorry :o ) with Rag_muffin as buying cheap brushes to test is just a waste of money. Your brush is your main tool that you should look after and you should always get the best you can afford, and for my money that is the master artist 508!
HTH