Hourglass launched this brush recently and it's my favorite to use with their Ambient Soft Glow Foundation. I also love using it with other liquid foundations – it's perfectly dense and fluffy.
This brush works best with Merit's products – specifically their foundation, blushes, and bronzer. But I love it for blending out other cream products, too!
If you're on a budget, Real Techniques makes amazing brushes. I bought a random brush set from TJ Maxx years ago and the concealer brush (like the tiny one in this set) is my go-to for spot concealing.