Firstly - there's no such thing as "the best push-knife". That being said, there's much less difference among push-daggers than among conventional knives - they all use the same grip, are all basically shaped the same, and are all used the same way. For that reason the budget knives (like Cold Steel's stuff) probably work as well as the high-dollar stuff (Mercworx, Wilson Tactical, customs). In fact, many people think that the thick, rubbery handle on the Cold Steel is more comfy and secure than Micarta or plain metal handles. I got rid of my Cold Steel Safekeeper because I'm a push-dagger snob and was saving up for a Mercworx, but ended up getting a Benchmade CBK, wrapping the handle in paracord and replacing the supplied clip (which isn't bad) with a mini Tek-lok.
The biggest disadvantage of a push dagger is that, unlike most small conventional fixed blades, it is difficult to set it up for an easy ambidextrous draw. If it's a horizontal belt mount, for instance, you can only draw it from one side. If it's vertical belt mount, you can probably draw with either hand, depending on where it is on the belt, but the draw is awkward with either hand because you have to pull straight up which requires articulating your wrist quite a bit. Since I want all my SD fixed blades to draw easiliy with either hand, I only carry my CBK when I'm hiking or hunting and want a knife that is very light (I mount it upside-down on the front strap of a backpack which means an easy draw with one hand, and awkward draw with the other - about the best you can do).