One of my go to products would have to be eyeliner. I just feel better when I have it on. Its just like mascara for me. I usually won't do one without the other, unless I am in a super big rush. I am not a huge fan of liquid eyeliner, I find it very difficult to use accurately. Specifically when I am working with clients. I much prefer to use an eyeliner pencil. I think they are less harsh than a liquid liner, easier to use, and look more natural - for the most part.
Finding a really good quality liner is pretty difficult. Every brand out there has an eyeliner selection so its hard to know what is good and what you're just wasting your money on. And just because it is more expensive, doesn't necessarily mean its better, either!