To get the highest rating on a stage you must complete it with all coins
left. Here is a tip on how to make life as a delivery boy a bit easier: 

Begin each stage by killing all the enemies instead of delivering pizzas.
Since you start with no coins you can not lose any, even if you touch an 
enemy! When all enemies that can be killed are gone you can begin to 
deliver the pizzas and earn your coins. With no enemies left you have
minimized the risk of losing any coins!