I think the Marbled Goby is very under-rated fish. They can reach over a foot and are voracious eaters. people dont get them because they are more active at dawn/dusk and at night. Mine will come out at about 5-7 because he knows when feeding time is. My girlfriend likes to feed him, because he will attack her finger. (shes twisted)
He has a tank all to his own with subdued lighting only from a tank next to his giving it a dark water feel.
I also agree about the pleco. Wet Wiskers caught an awesome one in a canal in FL. and it was pretty nice.
i think knives are very underrated, especially african brown knives. they are readily available, don't require much special attention and are really cool
I'd have to go with the Managuense. This fish might be very popular buy I never hear much about it. I just see some amazing photos of a beautiful one all grown out.
I wanted to add that i think african lake malawi mbuna are very overrated. for the price, they offer some of the most amazing colors and they have their own social ladder including alpha males just like a wolf pack. How cool is that?