Brush choice is very much a personal thing. When I first started out I didn't think that much of it -- I honestly thought those cheap-*** brushes you got at Michael's or Hobby Lobby were great.
Experience is a cruel but efficient teacher I suppose.
So nowadays my brush of choice for pretty much everything is a venerable Winsor & Newton Series 7 Size 0. It's such a versatile brush, good for covering (small-sized, admittedly) area and for detail work (down to painting in the creases between power armor plates). I wish I'd listened to the advice I was given oh so long ago and gotten one of them first...but I don't think I would appreciate it nearly as much as I do now if I had.
What about you guys? What brush(es) do you use? Have a favorite?