Kolinsky Sable is the best quality, normally used for watercolours and excellent for well diluted acrylic such as
GW paints. Mongoose should also be considered as it is also excellent for acrylic paint. Firmer than Sable, it is also more resilient and that may suit your painting style more.
If you can order online and don't mind the wait, I recommend "Rosemary & Co" series 33. The points are sharper than
GW brushes by a country mile, and the bellies are fuller so they hold more paint.
This is a quick picture of the ones I got recently. From top to bottom,
Rosemary & Co series 33 no. 2
Rosemary & Co series 33 No. 1
GW general purpose brush (Orange end)
Rosemary & Co series 33 No. 3/0
GW Fine detail brush
Rosemary & Co series 33 No 10/0
The only brush out of these to have been used before this picture was taken was the
GW general purpose brush, the darkening of the ends is an artifact caused by taking the picture on my portable DVD player screen so I could get them almost in sillouette to better show the point and belly of the brush
Take a look at
https://www.rosemaryandco.com/ to find out more.
Don't forget, how you look after your brushes is almost as important. Get a decent brush roll to look after the points, bamboo helps natural hair to dry and therefore helps resist mildew and mould if put away damp.*
* I am in no way affiliated with this company, I get no incentives from them, I just want to share their quality as I think they can't be beaten.
I tried D&R brushes before getting Rosemary & Co ones, and wasn't that impressed.