You forgot anableps, seahorses, goodeids, mosquitofish, and North America's smallest vertebrate, Heterandria formosa.
From your choices, scan down to
this thread where you will see a recent poll asking the exact same question.
As far as what is most interesting, if you just want pretty fish that will reproduce and require no further care or maintenance by the fishkeeper, then platies, mollies, swords, and guppies are it.
If you want a fish that requires some care and patience, try goodeids. For the advanced masochist, anableps are the peak of freshwater livebearer challenges.
Likewise, most successful seahorse breeders use nothing larger than a 20 gallon tank and an aquaclear mini for filtration.
What do YOU think is most interesting? Our opinions don't really matter in the end.
Good luck - don't limit yourself to you LFS, find a local aquarium society and talk to people. Make it a hobby!
/Kris
