I like tanks with ample swimming room, good filtration without too much flow, and a heater for a betta. I kept my betta with guppies and an amano shrimp and he left them alone. It all depends on the temperament of your betta.
so true, i have one that is unbelievably chill with tank mates, just not too "flashy" ones. my other one is... well he has some anger issues.