For what it's worth after so much good input here...
I started with sythetic brushes.
Then moved to off-brand Kolinsky Sable.
Then to Da Vinci Maestros.
Now I use W&N Series 7 Miniatures.
Each of those transitions was eye-opening, with the difference from synthetic to sable probably the most pronounced. The Maestros are good brushes, but I prefer the amount of snap, the paint flow, and especially the point of the W&Ns. Disclaimer: I paint with pretty thin paint - I'm not sure how the designed-for-watercolour S7s would do if you like to dip straight from the bottle, or only thin slightly.
I ordered my Series 7s in 3 sizes - 000, 0, and 2. I don't use the 000 - it's too small. Pupil dotting is actually easier with the wider belly and fine point of the 0. I really need a larger brush than the 2 for large blocks of colour. I should note that the Series 7 Miniature 0 is about the same size as a Da Vinci Maestro 000.
Finally, I got my latest brushes from Jerry's Artarama (although I believe Blick is slightly cheaper). I've paid much more for far inferior brushes.
Having the right tools for the right job really helps - but remember, you can't always blame your brushes! Good luck with the painting!