The Sigma F80 (formerly SS197) is my all-time favourite brush for foundation application. This is a flat top kabuki brush that is soft, yet firm and does a great job at putting on and buffing liquid foundation. This brush retails for $16 on the Sigma Makeup website. I wrote an initial review of this brush when I first got it, comparing it to the Everyday Minerals flat top brush, which is very similar, you can check that out here. I wanted to use it for some time to really test out if I liked it and post a proper review. If you are going to get one brush from Sigma, I would suggest it to be this one, I can't say enough good things about it.
This brush is very densely packed and is made of synthetic bristles, which makes it perfect for foundation application. I use this brush everyday and it has replaced my ELF Studio line powder brush which was my previous favourite, I still like it, but I find the Sigma one works a bit better.
Do you have the Sigma F80 brush?