Not a cory - they are schooling fish - and not a molly in a 5 gallon as that's to small for them, they grow to 4 inches.
I would actualy say 10 gallons would be better but if you are realy after a small 5 gallon-type tank, good options are guppies or platies (as mentioned). 3 of either or a combination but make sure you only get males as you don't want fry in such a small tank.
Another easy option would be a betta with a couple of otos or a large snail. Otos are rather fragile though and must have live algae to thrive.
4 or so endler's livebearers (only males again) would be beautiful and, being small, you could keep quite a few.
If you can find them, mosquito fish are easy and tiny but, again, they are livebearers so you may get unwanted fry if you aren't careful.
Lastly, a trio of honey gouramies (not dwarfs) would do quite well. Keep one male and 2 females as the male is territorial and will chase a single female constantly, causing serious stress. They aren't the hardiest fish but they aren't difficult either - and they are deffinately interesting and beautiful.