Which of these fish should I get?

I have a 55 gallon aquarium with a 70 gallon filter. I don't know which fish I should buy. I was looking at 3 fishes, comet goldfishes, black mollies and oscars.I already know about overstocking (because I have before with a 20 gallon and 5 goldfish before many years ago) so I want to make sure that this one is better. Since comets and oscars get so big, I'd only be able to buy 1 or 2, but with mollies, I heard I could get 10 or more. What do you think? What fish do you prefer?