I never leave the house without at least some makeup, at the bare minimum I will fill in my eyebrows (otherwise you just won't think I have eyebrows at all, its kinda scary) and some eyeliner. Without these two products I look pretty washed out and/or dead. I've been doing this since junior high school, ever since I discovered makeup and started tweezing my eyebrows. The only brow filler that I use, and will faithfully continue to use forever, is Clinique's Brow Shaper, which is a pressed powder that comes in a mirrored case with a small brush. But I have experimented with alot of different eyeliners, mostly pencil and liquid.
I use liquid liners mostly at night if I'm going out. I think liquid gives a very sharp look, very dark and "filled in", about as much made up as you can get with liners. The one that I have been using for years, and probably won't ever switch to another one, is L'Oreal Lineur Intense in Carbon Black. It's a felt tip liquid liner with a fine tip that allows you to customize the intensity and the width of the line you want to create. I have heard people say it's a dupe to Lancome's Artliner, which I have also tried and really like. They both look similar in terms of packaging, and pretty much creates the same effect, I honestly can't tell the difference. The reason I use the L'Oreal is that it's much cheaper, under ten bucks. I don't like to spend alot on products that I have to replace constantly (like mascaras they should be replaced every couple of months) and this is the best drugstore liquid liner that I have tried. It's just really easy to use because it's shaped like a pen. (This is also the reason why I don't really do cream/gel shadows, it is easier to apply with a pen shaped liner, and I think you still get a similar intense look with cream/gels as you do with liquid).
For day I tend to stick with pencil eyeliners because the results are less harsh than liquid. For a long time I was faithful to MAC's Engrave, it's also very dark, and is a good basic pencil eyeliner, not very expensive. But after receiving a sample of the Urban Decay 24/7 liner in Zero (came with the Naked palette) I think I'm going to be using this one alot more. What I like about the UD Zero liner is that it glides on so smoothly and I don't feel that I am tugging on my eyes, like I do a bit with Engrave, but I don't think it's as dark. I have also tried Make Up Forever's pencil liner, but I felt that I was tugging alot on my eyes, therefore I don't like this one. I have yet to try Smashbox's pencil liner, if I end up liking this more I will do a post on it.