Platies are my ABSOLUTE favorite fish EVER. My favorite is the mickey moush platy. They are VERRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRY hardy!!!!!! I didn't have too much luck with the sunset platies but my tank was screwed up(long story that I won't get into). So if my tank was scewed up and my mickey mouse platys lived for a year( moved and got rid of them so don't know if they are still living).
I really like the blue wag-tail ones myself although I only see them when i don't have room for them in my tank!!!
Good luck on your platies! They are SOOo fun to watch!!
I personally find that platies are very hardy. Which ones at your lfs appeal to you the most? If you want to have babies get one male and two females. If not get 3 males. Quite often the females can come from the lfs pregnant.