Zzzz. Someone needs to just make a stickied topic on this issue.
Mollies and platys are not good tankmates for a betta because they have different water requirements. They like water that has a higher salinity and hardness than bettas--while bettas can adapt to these water conditions, it can have a very negative effect on them, particularly their fins, over time. Mollies and platies can also be very nippy and aggressive toward bettas, and because of the coloring of platies, it can go the other way too because the betta might confuse it for another betta and attack it.
Pygmy cories are a good choice, but as was mentioned, they need to be in groups or they become stressed and don't live as long. African dwarf frogs can also do well, but they can take more care than you realize, so research them carefully to make sure you are up for it.
Some bettas simply don't tolerate other fish, keep in mind that domestic betta splendens were bred selectively for aggression before they were bred selectively for looks.