It depends on if you are building a flock or just keeping some chickens from dying while you own them.  If you are building a flock, then regular feed and judicious culling of any weak birds, without the crutch of medications, is the ticket.  Healthy immune systems are the only real defense against any disease.   
If you are just keeping some chickens and you don't want any to get sick or die, then medication is the way to go.  Of course, this doesn't keep them from getting sick or dying, but it will make you feel much better about your efforts if they do. 
I don't use medications and haven't had any sicken and only one die, but she had a blood clot in her pulmonary artery, so I guess that doesn't really count.  
Starting chicks out with an unnatural chemical in their feed or water doesn't allow them to develop a normal immune system.  Raising a bird with a fragile immune system is like waiting for the other shoe to fall.  
It would be interesting to take a poll on here of all the people who post about illnesses in their flock and find out how many of those folks started out with vaccines, medicated feeds, antibiotics, etc.