I would say it depends on what your painting with. If your painting acrylics and you seriously water them down then any top range Kolinsky sable i.e W&N Series 7, Da vinci, rosemaryandco just to name a few. If you dont water paint down then you can get away with cheaper Sable Mix Brushes, I use citidel brushes but am looking to upgrade, the army painter do a nice set of brushes which are same price as citadel but are better quality, W&N Galeria are a sable mix. If your painting enamals then i would go for a more expensive synthetic brush over the cheaper ones. The chemicals in enamels just destroy natural brushes, dry them out and make them brittle. But as i said you want more expensive synthetic, they will last better and hold together almost as well as sable. I use W&N series 7 for all my fine details on my Fantasy & 40K, But swap over to citadel brushes for my grunt work, as i said before im looking at army painter atm as there ment to be better quality.
Best advice really is decide what your painting with and then go out and pick up 1 or 2 brushes from the diffrent ranges, paint with all of them and see what feels right and works for you. Its the same old argument with paints as well. The products should work for or with you not against. once you know what you like go out and put together a set that covers all your bases. Oh and rember to use the biggest brush possible to get the job done, i.e dont use a 4/0 to paint the side of a hull, use a 5 or upwards.
hope this helps a little
Matt