Choose the ones that look the coolest to you.
I believe mollies need a larger tank (30 gal
or bigger) and they like some salt in their tank. description of mollies
Platties are cool colors and they only need a 10 gallon tank. No need for salt to speak of. description of platties
My grandmother has red wag platties and the babies are all a bright orange with a tinge of black for fins. So cute. Every one who comes to see her comment about the babies.
Guppies require a 20 gallon tank. Guppy babies would be harder to see because of their coloring, but they sure have neat tails when older. guppy description
Swordtails need a 30 gallon tank. Other than that I don't know much about them. swordtail description
In all choose which one you like the best and according to tank size if necessary. You'll be very pleased with any of them.