WWII: Which US Fighting Unit Was Best?

LTC Ivan Raiklin, Esq. I would the offer 5307th Composite Unit which was a United States Army long range penetration special operations jungle warfare unit operating in India nd Burma otherwise known as Merrill's Marauders after their Commander Frank Merrill.
"The 5307th Composite Unit Provisional (CUP) was a combat unit of less than 3,000 men from the 50 states. Everyone a volunteer, for what is now known to have been a long range penetration behind enemy lines suicide mission. These patriotic volunteers that answered President Franklin D. Roosevelt’s worldwide call for a dangerous and hazardous mission didn’t know what, or where, only that the expected casualty rate was 85% or more and their Country needed them.
• They operated behind enemy lines with only what they could carry on their backs or mules.
• They hacked their way through more than 700 miles of the World’s most difficult jungles.
• They repeatedly attacked the enemy’s infrastructure while spearheading the Chinese advance.
• Servilely outnumbered, they out fought the best of the enemy’s 18th Imperial Marines.
• In the five months, they fought five majors battles and thirty minor ones.
• In monsoon rain they cut their way over the impossible 6,000 ft. Kumon Range to Myitkyina.
• They captured the only all-weather airfield in Northern Burma, at Myitkyina.
• Endured for five months well beyond the known level of human jungle endurance.
• No other unit in annals of military history ever fought so long under such adverse conditions.
• With only what they could carry on their backs or mules and resupplied supplied by airdrops.
• Living on inadequate emergency ration they continually fought malnutrition and exhaustion.
• Suffering from fevers, diarrhea, jungle rot, fungus, leach infections, was the norm.
• They cleared the enemy out of Northern Burma and enabled a land route to China.
• Through perseverance, tenacity, and comradery, they unexpectedly managed to survived.
• After a thorough war record search, the 75th Rangers selected the 5307th CUP for their heritage.
• 25 Marauders, for their outstanding heroic action were inducted into the Ranger Hall Of Fame."

https://www.youtube.com/watch?v=RkRbX2obs3c
If you broaden this tow Allied Special Operations forces I would add Office of Strategic Services (OSS), Royal Commando, Chindits and lastly the ten Gurkha regiments, with two battalions each making a total of twenty pre-war battalions.
COL Mikel J. Burroughs LTC Stephen C. LTC Greg Henning LTC Ivan Raiklin, Esq. Capt Seid Waddell Capt Tom Brown CW5 (Join to see) SGM David W. Carr LOM, DMSM MP SGT SFC Joe S. Davis Jr., MSM, DSL SFC William Farrell SSgt Robert Marx TSgt Joe C. SGT John " Mac " McConnell SP5 Mark Kuzinski SPC (Join to see) SrA Christopher Wright PO1 John Crafton CPT Gabe Snell

IMHO, all the units that fought during WWII were of equal praise.

Easy answer --- 100th BN 442d Infantry. Go For Broke!