The Lancome artliner in black is the one I turn to at all times for a gorgeous liquid line, or a smooth cat-eye. I used to use Maybelline's similar liquid liner (a steal for under ten dollars) but when that either went off the market or out of my area, I switched to Lancome, who had a very similar applicator. To be honest, it's the applicator that sold me. It's a non-rigid penlike tool that isn't "brushlike" at all, helping keep the line uniform. It can produce a very thin subtle line, or it can go much thicker depending on how you hold it and use your hand.
I wear mine almost every day, and each tube lasts 6+ months, which helps me justify the expense. Yes, it is pricey, but it's high quality. It doesn't smudge on its own, unless I rub my eyes or itch the corners or something, which I do tend to do with my contacts and all.
This layers well over eyeshadow as well, and can be layered upon if you don't want an obvious stark line on your eye. As one of my staples, I'd recommend this to anyone.