A lot comes down to the betta's personality, but in general, gouramis, brightly colored fish, and long finned fish sometimes trigger aggression in bettas. Any fish with even the slightest nippiness in it may not be able to resist having a go at the betta's fins, even if they don't bother any other fish (for example, neon tetras).
Often recommended tankmates: Otos, corys, white cloud minnows.
Betta's are a good example of why a straight unqualified list of community fish is prone to problems. They can be good community fish, but usually not with many other good community fish. Angelfish and tiger barbs can both be kept in a community, just not the same community.