I don't know why I only have one bent eyeliner brush in my collection, the Smashbox Arced Liner Brush #21 you see here. It's a good and efficient tool for gel, liquid and cream eyeliners, and one that is wonderfully easy to use. As a matter of fact, this is the brush I often recommend to liquid/cream eyeliner beginners, and have also bought it for my mother-in-law when she ventured outside of her makeup comfort zone.
When it comes to size and proportions, Smashbox Arced Liner Brush #21 is right between MAC 209 and 210. It's a case of not too wide/not too narrow, very easy to control and create a line as thin or as thick as one desires. The angled shape of the ferule ensure that you can get right into the lash line while holding the brush in a very comfortable grip that allows you to see exactly what you're doing and where you're going. This is why it's such an essential brush for eyeliner novices- Smashbox #21 takes out a lot of the guessing from eyeliner application. More experienced users will be able to create a super thin line (just like the one you will get from the Sephora #19, which is great but not as easy to use. By the way, Sephora does offer a similar brush, Professionnel Platinum Angled Eyeliner Brush #23. I might pick it up eventually as a backup).
Smashbox is a PETA-approved brand and all brushes are cruelty-free. The hair of the Liner Brush #21 is made of a "sable substitute", whatever that is. In any case, the head is finely shaped and has the necessary firmness for precision work without poking you in the eye.
Bottom Line: an essential
Smashbox Arced Liner Brush #21 ($20) is available from Ulta, Sephora, Nordstrom and smashbox.com.