The things I’ve used most in my chicken medicine cabinet are calcium plus D3 and triple antibiotic ointment (w/out pain relief). Doesn’t hurt to have some plastic gloves, gauze and vet wrap. Betadine, Vetericyn or chlorhexadine come in handy, but you can easily make sterile saline from salt for rinsing wounds. Epsom salts are good to have on hand for soaking wound or egg binding. Mites have been bad this year and I have benefited a few times from having a permethrin powder on hand.
Most other things in my first aid kit such as antibiotics will probably expire before I get a chance to use them, so I don’t think I’d buy them again unless I needed them.