There will be some smart alec answers that go along the lines of "clean water" - and in the main I'd agree (does that make me a smart alec too?). Many meds have a shelf life so you have to weigh up the cost of keeping meds that you hopefully will never use only to throw them away or worse still when you come to use them they are ineffective because they have expired.
Generally I have PP crystals on hand and chloramine-t. Antibiotics I get as and when needed, that way they are fresh. I get wormers as needed too. I will sometimes have F&MG on hand, but usually end up throwing it as it does lose it's potency once opened, btw never keep formalin products in a fridge in case you think it may prolong it - it won't - it will create a toxin called paraformaldehyde.