You'll want to know what is rampant in your area and what isn't likely to be a problem.
The Core vaccines are rabies, tetanus, E/W Encephalomyelitis, and WNV.
These typically are a must.
If you travel or have a fairly regular change of boarders then you should look into strangles, Flu and Rhino.
There is also anthrax, botulism, and snake bite, and Potomac Horse Fever, but these are based on risk more so than anything.
I personally like to have all, except the anthrax and snake bite, as it's not too much of a risk for me. If you do no other vaccines Rabies should always be done.
"They see me rollin, They hatin, Patrolling they tryin to catch me ridin dirty"