The best brushes are kolinsky sable rounds Raphael 8404 and Winsor & Newton Series 7. Raphael's have longer bristles and fuller reservoirs, but are harder to find. Winsor & Newton's are more common and widely known, and feature a sharper shape with shorter bristles which provides a slightly stiffer tip at the cost of reservoir capacity and softness. Artus Opis and Da Vinci Maestro are also top quality sables. Rosemary & Co are acceptable quality kolinsky brushes at a more affordable price range.
If you prefer stiffer brushes, you'd need to use some kind of high quality synthetic, which I don't know enough about to comment. Even if synthetic may not be as optimal as kolinsky sable, if it's what you're more comfortable with, you are going to inevitably end up painting better with a snappier synthetic.