With liquid liner, what you're generally looking for are precision and lasting power (although being waterproof can be pretty great, too). Dior Intense Liquid Eyeliner dries really quickly, which makes application a whole lot easier, and the narrow brush makes getting a fine line superlatively easy. For something a little more portable, Hourglass Precision Liquid Liner comes loaded in a pen and has a similarly versatile brush that's slightly shorter, which makes for a less dramatic, more daywear-appropriate line.
If you're an advanced liner applier, of course, the rules are a little different. The classic for gorgeous vintage-style cat eyes is Guerlain Liquid Eyeliner, which has a super long, super fine brush that makes a perfect flick and comes in the prettiest bottle around.