I would probably have to say the 19th Indiana, the 19th was an original component of the famed Iron Brigade.
The regiment was formed in Indianapolis, Indiana in July of 61 and comprised of men mostly from the central part of the state. The 19th saw combat at many notable battles including Antietam, Fredericksburg, Chancellorsville, Gettsburg, Cold Harbor, and Petersburg. Through various sources I've found that during the war the regiment suffered between 298-327 fatalities during the war with roughly 2/3 of those deaths being combat related.
The 19th was integrated with and would become part of the 20th Indiana in October of 64.