I keep a few things on hand, some are things I think are essential, others are left over from previous treatments.
Salt - Kosher, pickling, table or aquarium (NaCl). Since Ich seems to be one of the most common illnesses in FW tanks, it's a good idea to always have some salt around.
MelaFix - Useful for mild bacterial infections, torn fins, minor wounds.
Metronidazole - This is an effective medicaton for intestinal flagellates like Hexamita and is also a mild antibacterial. It works best internally, I mix it with food. I've read that some discus breeders use this medication routinely in their quarantine tanks.
Clove oil I keep some on hand for euthanizing fish that are too sick to save.
That's it. If an antibacterial or antifungal medication is needed, I'd buy it when you need it - medications expire quickly, and certain conditions need very specific medications. If you're going to keep something on hand for bacterial/fungal infections, I'd get Kanacyn - it treats gram positive and gram negative bacteria and some fungi.