I use the brush recommended by the manufacturer, and I have a differernt brush for each liquid I use. Once my brush has been introduced to a monomer, they are friends for life and become dedicated until the death of the brush. Most good brushes run about $30 -35 usd and are well worth the investment.
Some things to look for in a brush, bristles that are glued as well as crimped into the ferule, a flag no longer that a quarter of an inch with a nice swell to the belly and hairs that are sleek and smooth. There shouldn't be stray hairs poking out. The major manufacturer's have spent all kinds of time and money researching to make sure that their brush will hold the correct amount of liquid for their powder so that you have product control.
I have brushes from almost all the companies, and as long as I follow manufacturer's directions, it's the best brush for the system. Having said that, my favorite is the master sculptor by Creative Nail, follwed by the EZ Flow 508 I don't use much OPI any longer, but they have some nice brushes as well. (I'll admit to having a brush fetish, and I'm a little obsessed with how they are cared for):o