We get this question from our customers a lot, and it's a little bit like asking a mom which of her children she loves the most. But, if pressed, we do have to answer with "probably the #39 brush." If you are looking for one eye brush that you can use to create a full eye look (excluding liner), the #39 can do it.
For all-over color, the #39 is perfect. Its broad-bristled end will pick up a good amount of color, and because the end is wide, it covers a larger area well and quickly. Then, if you want to do the crease or another smaller area, turn the brush on its side and use the narrower dimension to contour your crease or lid. To blend out the crease, flip it back to use the broader dimension. Add some liner and you are set to go!