I go through a massive number of eyeliner brushes every week, which means at any given time I must have several clean ones in various shapes at my disposal. That's why I tend to buy them from just about any of my favorite brands, hence the appearance of these two Louise Young brushes in my collection at some point late last year. LY23 and LY24 are of the traditional long(ish) and fine school of liner brushes. They're both made of sable hair and meant to be used with liquid, cream and gel eyeliners, as well as dampened for cake liners (or eye shadow).
LY23 has a wider base and tapers towards the tip, similarly to MAC 209 though considerably smaller. Size-wise, both these Louise Young eyeliner brushes are close to the MAC 210. LY24 is a very fine but not nearly as fine as my old Sephora one that's now discontinued (I don't know if their Classic #17 is as thin as the Platinum Pro I have). Choosing between the two depends on the eyeliner's consistency (the runnier it is, the easier the wider brush would be) and the desired effect. I like and use both on a very regular basis and I'm happy to report they've been holding well (for comparison, look at the poor mangled brush I picked who-knows-where in some kind of an emergency around the same time).
Bottom Line: valuable basic tools.
Louise Young LY23 (£9.00) and LY24 (£11.00) Eyeliner Brushes are available from louiseyoungcosmetics.com. They ship worldwide.