Since graduating from kohl a couple of years ago, I have sampled my fair share of liquid eyeliners: pens, pots, gels, thin liners and thick liners - I've tried them all.
This Paul & Joe Liquid Eyeliner is without a doubt the best one I've found to date. A pen-style liner, it comes in black or brown and is the only one I've ever been confident enough to apply on public transport. The nib is the perfect thickness and the awesome consistency prevents gloopy mistakes, making it easy to obtain a smooth line and flick.
It also has a 'clicky' lid (you'd be surprised how important a 'clicky' lid is to me), meaning it doesn't dry out super quickly and seems to last a while.
You can find it on Asos for £18