Each kind has its own unique qualities. I guess it would have to depend on what your're looking for, and your personal taste.
Me, I liked guppies first because they were friendly and unafraid of anything. They will come meet you at the side of the tank. Because of this, they are also easy to net. It is interesting to breed them and see what you can produce. Unless you get good stock that breeds true, this can end up in a lot of culls, though. Due to small size and large, colorful fins of the fancy varieties, they are vulnerable to other, nippy fishes.
I got into platies next because I heard they were hardier than guppies. I found blue platies cute, also the orange Mickey Mouse pictured in the link above, as well as hifin varieties. Here are a couple of links with some of the colors available:
http
/www.tropicalfishintl.com/platy.htm
http/www.jackspets.com/freshwatermoon.htm
Mollies I never kept, due to their being more brackish. Also, they and swordtails seem to tend to be larger (ergo take up more tank space) and prone to bullying based on what I hear from others, but I could be wrong as I have never kept either.
If you're looking for variety, I suggest you consider choosing one or two color/tail varieties of each type of livebearer you like (assuming they can coexist all in one tank), so that there won't be interbreeding of different varieties, unless that's all right with you.
Anyway, since you're looking for pics, here's the link for some of mine (guppies and platies):
http/photobucket.com/albums/v196/nicoandfriends/Fishes/
of livebearers to start off my 40gallon tank 
)
/www.adaniels.com.ar/enciclopedia/Du...ickey_Mouse.jpg
/members.iinet.net.au/~cichlid/swordtails.jpg
/www.aquariumfish.net/images_01/guppy_male_blue.jpg
/www.aquariumfish.net/images_01/dalm..._molly_w270.jpg
/www.aquarticles.com/images/endlers1.jpg
