I think that you should use the Sephora Collection Classic Angled Liner/Eyeliner Brush #15. It is perfect for winging out liner and is also made with synthetic bristles so it will apply smoothly.
The angled liner brush is the most universal brush for gel or cream liners. You can get a different brush if you know you will only be doing one specific look, but the angled liner brush can do most any line, from thick to thin and even a cat eye. This is a good staple liner brush to start with. Then you can get different ones for different looks as needed.
The Sephora Collection Classic Angled Liner/Eyeliner Brush #15 ($14) is a perfect example, but there are other brushes out there for less if you don't want to dish out the cash on an experiment.
Hope this helps!
I use a watercolor brush from an art store, it cost around $8 and has lasted years. Has nice synthetic bristles and a great pointy shape that gets into my eye corners very precisely.