No barbs or cichlids (or other well known fin-nippers), or aggression tagged fish. And probably not a good idea to put fish similiar to bettas like gouramis, or any fish bigger than them unless you're sure they're docile, because the betta will become the victim.
Just find some docile, pretty fish you like that aren't aggressive and preferabley not long-finned (for their body size). I did have bettas with some lyretail fish and other long-finned fish of their size, though.
Not all bettas are really aggressive, except to maybe other bettas. You can introduce fish with dividers or closely monitor them when adding fish. Monitoring is always a must for any introduction of animals, because they're unpredictable.
Also, make sure to have a backup plan for any fish you get. As perfect as they may sound, some fish may have a certain temperment that doesn't allow them to be with other fish - even if it goes against standards for the species.