Definitely platies. Mollies and swordtails are too mean to my bettas and my bettas seem to get harassed by guppies due to their constant attempts to breed with the bettas. Platies leave my bettas alone and are super sweet. I can have colorful platies without my bettas reacting to them as well, while swordtails might be targeted. It's funny, actually. My bettas have eaten guppy fry before, yet they leave the platy fry alone. Platies galore! xD