Aquarium salt and Epsom for sure but for actual meds:
API Furan 2 - treats both gram negative and gram positive bacteria such as columnaris
API General cure for parasites
Quick Cure (malachite green) for ick and fungus
I also have: API Tetracycline - which is also an anti bacterial med.
Jungle Anti Parasite fizz tabs - treats external parasites
Jungle Anti Parasite pellets - Internal parasites but bettas won't eat them
Krodon's Fish Protector - helps heal fins and scales.
Amquel Plus - detoxifies Ammonia, nitrite and nitrate
Mardel Maracyn 2 - Anti Bacterial for gram negative bacteria.
You do not NEED all that stuff. Most of it is leftover from when I lived in Alaska and did not have easy access to the API meds. The Fish Protector and Amquel plus I used for their water when I moved from Alaska to New York in January. The fizzy tabs I got to make my own medicated pellets using their actual food. Amazingly enough, it worked. Maracyn 2 is popular but I had to OD the fish on it to get it to work...supposingly if your PH is too high, its not as effective.
There is a whole bunch of crap you can get but I would just get API Furan 2. You should have something on hand in cause of columnaris which can kill quickly and Quick Cure for Ich and velvet.
This site lists a few different meds and what they are for.
OH - do not get bettafix or anything ending in FIX.