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!