Does anyone have a list of the best zone drops for every coverage in the game you should set?
This is a tough question to answer since zone drops are set as a response to what your opponent is doing. For example, if they're throwing bubble screens, 0 yard flats from the inside will stop it (requires a pass commit vs. RPOs). If they're throwing a corner route, 10-20 yard flats are the answer depending on the depth of the corner. Some special corner routes are even deeper and require 25 or even 30 yard flats to take them away.
One tip that seems counter-intuitive is that you want to set your flat zones deeper than your curl flats. A lot of pros will set their curl flats at 5 and their hard flats anywhere from 15-30. This is because flat zones will play the sideline much better than curl flats.
There's no one answer to set all your zones to and then forget it. You should be using them to defend areas your opponent likes to attack.
Last thing, remember that setting any zone drops at all will turn off match coverage. If you call a play like Cover 4 Quarters but you have any zone drop set to anything other than default, the match zones will convert to spot drop zones.
Like Rocketz said, it really depends on what your opponent is doing... If they're throwing those deep crossers and corner routes, you'll probably need to set your flats to 25 or 30 yards, but if they're throwing a lot of slants, set them closer to 10-15. Once your opponent shows you a few plays, start adjusting your zone drops to try to stop his favorite routes.