Kabuki brushes used to be the standard round brush. Now they come in various types, including: flat top, round top, angled top and tapered top. The idea is the same, but the application processes and usages differ slightly. Unless you’re a professional makeup artist or some sort of odd brush lover (raises hand), you will not need all four brushes.
As evident from the fact that I have three flat top kabuki brushes in my often used brushes selection, the flat top kabuki brush is my favorite kabuki brush. (For the Sigma brands, that is the Sigma F80 brush, which roughly translates to the MAC 180 and MAC 183 – both discontinued.) The flat top brush applies makeup quickly, evenly and thoroughly. It makes a smooth application and finish simple. If you want one face brush, let it be a flat top kabuki brush unless you use a lot of mineral makeup. The rounded kabuki works a little better with a rounded kabuki brush.
The angled kabuki brush is convenient for contouring the side of the face, but it isn’t anything a flat top brush can’t do. The tapered kabuki brush is a total splurge item, in my opinion. It’s unnecessary and not something I ever reach for. It’s purpose is to reach hard to reach nooks, but I just use smaller brushes for that.