Luke, they are all good units and you can't go wrong with any of them. If my Vexilar Fl-18 ever goes out, my next purchase will be the Humminbird Ice 55. It has features that the Vexilar doesn't have, such as a digital depth gauge, 6 colors, and a moveable zoom. The thing that seals the deal for me on the Bird is the digital depth gauge. I am usually drilling a bunch of holes and hopping between them, and don't have the patience to do the math to figure out depths and trying to remember which scale to use. I believe that the Marcum LX-5 has more power and a better interference rejection system, but unless you are in a crowd with other flashers this really isn't an issue. Not sure if this is true or not, but I've heard that the battery run time isn't as good on a Marcum as the other two, simply because it puts out more power. When jigging, the Vexilar's seem to be the smoothest. When watching the jig move up and down on the flasher, it is a smooth transitiion, whereas the other two manufacturers, the jig may seem to bounce a bit between depths.
To make a long story short, you will want a flasher that gives you zoom, good target separation, is easy to use and will be durable. All of these units will do that for you.