The most decorated unit in the U.S. military were... Japanese 

100th Battalion, 442nd Infantry - made up entirely of 'Nesei' (2nd generation Japanese-Americans), even as the U.S. held thousands of Japanese, including their family members, in interment camps.

 "One Puka Puka" - 100th Infantry  (The "Hawaiian Provisional Battalion")

The 100th/442 fought in Italy, southern France, and Germany, in some tough campaigns including the rescue of the "Lost Battalion" (141st Regiment), and even captured a German Submarine!  In all, 9,486 Purple Hearts were issued (many earning double and triples), and the official casualty rate, as a fraction of all who served, is 93%.  It was awarded 7 Presidential Unit Citations, 4,000 Bronze Stars, 1,200 Oak Leaf Clusters added to the Bronze Star, 560 Silver Stars, 28 Oak Leaf Clusters to the Silver Star, 52 Distinguished Service Crosses, 15 battlefield commissions, and more than 20 soldiers were awarded the Medal of Honor.