I really like this one:  http://northamerica.hardi-commander.com/
http://www.hardi-us.com/upload/pdf/usa/products/trailed/new%20commander-4400%206600%209000%20.pdf

 Not cheap, but great resale and durability.  Loaded with features and the Force boom is the best I've seen for stability and wear.  I would buy a used Hardi over new of many other brands.  It all boils down to how many acres you are sending through a sprayer a year and what you want it to do.  (i.e. high/low volume, high/low pressure, induction, capacity, tractor size, etc.)