A very good list and link from
@ChickenCanoe
Making plans and being prepared goes a long way.
Something else to think about is
where and how you will keep a sick or injured chicken. Having a large tote or wire dog kennel on hand is always a good idea. Sometimes a chicken may need to be separated completely for extra care, other times a kennel set up in the run or coop will be sufficient.
If you have space, you can also just have an extra shelter built to house a chicken or 2 that needs to be separated. I use my grow out area for separation, but a kennel comes in handy when I have babies and the pen is in use.
One or two other things:
epsom salts
a dedicated dish pan - you will find this comes in handy for a number of things, from washing a poopy rear end to soaking an injured foot.
For most of your medicines/supplies, put those in a small see through tote with a list so you know what you have on hand.
Oh! Don't forget rubber gloves and a few face masks too!
LOL One more thing - it never hurts to have some poultry vitamins like Poultry Nutri-Drench or similar on hand - these can be used to perk up a chicken as well as give the whole flock a boost every now and then.