As mr86mister says, no other bettas, tetras, barbs, or danios -- danios are fast swimmers and they need a bigger tank anyway. No guppies -- they are brightly colored like the betta, and the betta may think the guppy is another betta and attack.
Actually, in a 5 gallon tank, I wouldn't recommend any other fish. Cories may be ok with a betta, but they need to be kept in groups of 3 or more and you won't have enough room in the 5 gallon tank. I have two bettas, each in their own 5 gallon tank. This tank is the right size just for one betta. If you put more fish, in addition to the one betta, in this tank, you will most likely overcrowd the bioload of the tank, and cause aggession among the fish because they will not have enough space for their own territories.
The only other possibility I can recommend is some ghost shrimp. Some bettas will get along with the shrimp, and some may eat the shrimp. It's probably a 50-50 chance, but you can try. I had some shrimp with my bettas once. One betta was very curious and didn't eat the shrimp, but wouldn't leave it alone. It died after about a week. The other betta totally ignored the shrimp in his tank.