Fine Liner Brushes
The classic fine liner shape is perfect for liquid liners, here is a Kryolan synthetic fine liner brush (that is super bent--sorry, I keep forgetting to fix that) and a discontinued NARS. I like to use them even when the liner in question has a felt or sponge tip. They draw a super fine line and are great for basic liner application as well as wings. I also like using them for gel and cake liner or dampened eyeshadow. If you want to do a really thick wing, I think you'll find it frustrating to use a brush this fine, and they're not known being beginner friendly, but still an important and useful brush.
Pointed Liner Brush
A thicker and often stiffer brush than the finer liner brush, the pointed liner brush, this one is by Laura Mercier, tapers from the ferrule, where it is round, to a thin point. It's a great brush for thick liner and it's size makes it more user friendly than finer brushes. It won't maneuver around the lashline as well as the finer brushes, though, but if you do "big" liner a lot, you'll probably like it better.
Gel Liner Brushes
The gel liner brush (Sephora's, a definer brush from Laura Mercier, and the Ultra Fine liner from Bobbi Brown) combines the best of both the previous brushes, it's wide at the pinched ferrule then tapers to a point, often making a triangle shape along the way. These are often sold as or packaged with gel liners, and they really are great for them. The unique shape offers both flexibility and stability, so they are my number one choice for beginners at gel liners. These are also perfect for using eyeshadow to line the eye, either wet or dry. Outside of the eye area, these brushes make for great spot concealing brushes.
Another style of gel liner brush ends in a rounded square shape; these are a soft Stila 4 and a stiffer Make Up For Ever 2s. They are also good for gel liner, especially the MUFE one, and I love them for powder eyelining. They are also great for blending pencils after they have been applied to the lash line.
Flat Liner Brushes
The flat liner brush is virtually a necessity if you tightline, which, in case you don't know, is to line by putting liner into the base of your lashes from underneath (it's not to be confused with lining the inner rim or waterline of your upper lashes). A cake liner and one of these babies and you are good as gold. They are also good for pushing liner into the lashes from above, especially a cream or gel texture or powder shadow. Either way they give an "un-made up" look with fuller lashes. These brushes are a Stila 13 (a little wide for my rounder eyes), a Laura Mercier, Paula Dorf, and an Ulta.
Angled Liner Brushes
Angled liner brushes are another category that multi-tasks (yes, that tiny brush on the far left is angled, it's by Sublime/Brandon, next to that is a Japonesque, MAC 266, Benefit Hard Liner, and Real Techniques Brow Brush). These are as useful for brows, if not moreso, as they are for eyelining. As brow brushes, they apply brow powder, powder eyeshadow, or cream and gel brow products to fill in and define the brows. The Benefit hard angle is my usual brow brush for use with powder eye shadows and the MUFE waterproof brow corrector. As eye liner brushes, they are commonly suggested for use with gel liner (I know MAC sales associates have often pushed it), and I have seen some great things done with them, but they do not work for me to apply my own gel liner and often apply too thick of a line for my tastes. I do like them better when used on someone else, and they are terrific for creating a wing at the outer corners (which is why I think they are often suggested). They are also good for applying eyeshadow as liner or for blending pencil. I think, as a brow brush they are essential, but as a liner brush, I only suggest them if you struggle with your wings.
Pros and Cons
Pros: One of these liner brushes should be in your brush collection. The fine liner offers the most control and creates a multitude of looks. The pointed liner brush works for dramatic looks. Gel liner brushes are (obviously) made for gel liner; they're great for beginners and offer both precision and ease of use. Flat liner brushes fill in the lashline, making lashes look thicker without a definite line. Angled liner brushes are superb for eyebrows and great for getting a good flick on eyeliner. Even if you only use pencil liners, you'll probably want one of these brushes for blending or applying eyeshadow to lock it in.
Cons: What fine liner brushes offer in flexibility they lack in ease of use. The classic "liquid liner is so haarrd!!" cliche is probably due to this brush. (I should probably clarify that I love this sort of brush and have used the two shown here practically to death). The pointed liner brush can be unwieldy in it's size and stiffness. Gel liner brushes are great, but I know some of them can splay and not offer a fine enough line. Flat liner brushes are essentially unitaskers, and they have the problem that they must be held perpendicular to the lash line, and if you're nearsighted (as I am), you'll likely hit the brush handle into the mirror. Angled liner brushes work for some people, but they don't work for me.
The burshes featured in this post are: Kryolan Professional Size 0 Round Details Brush (No. 4300), NARS Liquid Liner Brush (dc'd; I don't know how the current version compares), Laura Mercier Pointed Eyeliner (from a double sided brush), Sephora Gel Liner Brush #26, Laura Mercier Wet/Dry Definer Brush, Bobbi Brown Ultra Fine Liner Brush, Stila Precision Eye Liner Brush #4, Make Up For Ever Eyeliner Brush 2S, Stila Flat Liner Brush #13, Laura Mercier Flat Eyeliner (double sided brush), Paula Dorf Eye Definer (from a set), Ulta Flat Eyeliner Brush, Sublime Small Angled Shadow Brush, Japonesque 940 Angled Eyeliner (travel), MAC 266, Benefit Hard Angle Liner Brush (this looks different on their website, no idea if it's comparable), and a Real Techniques Brow Brush (from the starter set). All brushes were purchased and I received no comepensation for featuring them.