My favorite eyebrow brush in the last 6 months or so is RMK eyebrow brush with its delicate rounded edge of uncut hair. It gives me maximum control and very natural looking results. I do wish the handle was a little longer, but that's a minor complaint. I still think it's not a must have if you already have a couple of other good eyebrow brushes, but I can't deny it's the one I grab consistently.
However, there are several other good brushes for filling eyebrows. I've had some of these for years and have an idea why a couple of them are better than others. Do take note that my tortured and abused Sephora brush is a much older version of today's No. 15. It's from before they stamped the numbers on the handles and I'm guessing it's the #15 equivalent. It survived after all these years, but just like MAC 208, I find the hair to be too long for the purpose at hand and the edge is too blunt.
Paula Dorf eyebrow brush is another oldie, but a much better one. It's very balanced and the head's width and shape allows for even brush strokes that are less visible and more natural looking.
Shu Uemura 6 ob brush is very narrow. It's the one to use for tiny gaps and minute eyebrow corrections. The Shu Uemura brush feels very balanced in my hand (extremely important when dealing with eyebrows as everything you do can change your facial expression). It's probably a result of head/handle ratio.
My favorite in this bunch is Hakuhodo S163 (S100 series). The short hair ensures precision and since the head is densely packed with water badger hair that seem perfect for the task. Hakuhodo also offers other interesting looking eyebrow brushes, especially in the G series, but honestly, I don't see a reason to add another tool any time soon. Between the ones here and my RMK brush I feel like I'm all set in this department for the foreseeable future.
Sephora #15 ($14, in store and sephora.com)
MAC 208 ($19.50, maccosmetics.com and at the counters)
Paula Dorf eyebrow brush ($24, at Henri Bendel and pauladorf.com)
Shu Uemura 6 ob ($29, shuuemura-usa.com)
Hakuhodo S163 ($28, hakuhodousa.com)