If you must have a hang-on skimmer the one that I seem to remember being awesome was teh Remora. I will try to see if I can find out who makes it. You going reef or fish only?
AquaC remoras are a very good skimmer IMO. You will always find pros and cons but i think for a hang on skimmer it does a pretty good job. If you ever add a sump you can use it in the sump just the same to get if off your tank so you really cant go wrong.