I have them all. I use them all. Each of them is great in its own way.
(in the pictures I used: Liquid - Essence liquid eyeliner, Pencil -Black Long Lasting Essence pencil eyeliner, Gel- unknown brand Gel liner, Powder - Dr. Dima's Cosminilogy powder eyeliner)
It's very resistant and stays on for a very long time. If you apply it perfectly, it stays so for the whole day. The hardest one to take off. It may take a bit of practice to make the lines perfect. It's the one that gives you the most control. I can't use it in the waterline. Best color payoff. You need to wait a couple of seconds before it's 100% dry. Shortest shelf life.
Easy to apply, doesn't smudge so quickly. But it doesn't stay on for the whole day. It may 'print' itself on the crease. I use it for the waterline and bottom line.
Similar to liquid liner, except you need a brush to apply it. It may 'print' into the crease if you apply too much of it.
You need a brush to apply it. It's easy to apply, stays put on for a long time, but it gets smudged easily on the lower lash line. Not the best color payoff, it is the most subtle from all three.
When I'm in a hurry I use powder eyeliner, if I want it to be very precise and 'hot' I use liquid eyeliner (and bold), if I want it to be looking good, but not so bold and I'm not having all the time in the world I'd go for gel liner...
Pencil liners are not my thing, at least when it comes to the eyelid part.
Which eye liner do you prefer and why? Liquid, pencil, gel or powder liner?