I picked Smashbox Eye Definer Brush No. 15 on a whim at Ulta because I was looking for a small domed brush for my travel brush roll to do detailed crease work and maybe also a little lash line smudging. I liked Smashbox #15 because it's quite dense and the shape is very well-defined. The brush is almost pencil-like while still soft and pleasant to use. It fits right where I need it- in the crease or right above my lashes, so I find myself using it very often, especially for really dark colors.
What the brush doesn't do: It's not the right tool for blending, at least the way I work- too small and tapered for the task. I'd also avoid using it with a cream shadow, despite what it says on Smashbox website. The brush is made of natural squirrel hair which I think is too porous for cream products. There are enough synthetic brushes better for this task.
Smashbox Eye Definer Brush No. 15 ($24) is available from Ulta and smashbox.com.
All photos are mine, models are Sophie and Giselle.