If you happen to be in the UK, rush out and buy a copy of Practical Fishkeeping. I have an article there all about the less familiar livebearers, of which there are many! From 20 cm pike livebearers and four-eyed fish, through to halfbeaks and porthole livebearers, there are plenty to choose from. Tracking down the "oddball" livebearers can be tricky, but it's well worthwhile.
Of the "big four" livebearers, I have to confess to having a soft spot for mollies, particularly wild-type sailfins. Real sailfin mollies are huge: the biggest females are about 18 cm or so in length, and the males not much smaller, and when those males develop their fins properly, they're a real sight to see! But even common-or-garden black mollies are lovely fish. Whichever you choose, hunt out good stock. There's a lot of mass produced, very ropey fish out there. If in doubt get in touch with a fish club -- they're usually very good at putting you in touch with serious breeders. Livebearer aficionados are, in my experience, lovely people who are only to happy to share their stock.
Cheers,
Neale