i usually use mac eye kohl, and occasionally i use fluidline. i think the eye kohl stays a little better on me, but fluidline gives me a very clean, straight, smudge-less look.
in my experience though, a lot of people have trouble keeping products on the waterline, because of sensitive or watery eyes. if your having a lot of trouble with it, you may just be irritating your eyes. just lining your lashline may be better for you.