re: WWII Buffs – what was the most audacious action by a US unit in the war?

As to the OP, I really think the Taffy 3 action took the biggest balls of the war.

7 destroyers and destroyer escorts against 23 Japanese cruisers, destroyers and two battleships, including the most powerful in the world, the "Yamato".

The escort carriers and destroyer escorts which had been designed to protect slow convoys from submarine attack had been repurposed to attack ground targets, and had few torpedoes as they could normally rely on Halsey's fleet to protect them from any threats from armored warships. A Japanese surface force of battleships and cruisers, battered earlier in the larger battle and thought to have been in retreat, instead turned around unobserved and encountered the northernmost of the three groups, Task Unit 77.4.3 ("Taffy 3"), commanded by Rear Admiral Clifton Sprague. Taffy 3's three destroyers and four destroyer escorts possessed neither the firepower nor the armor to effectively oppose the 23 ships of the Japanese force, but nevertheless desperately attacked with 5"/38 caliber guns and torpedoes to cover the retreat of their slow "jeep" carriers. Aircraft from the carriers of Taffy 1, 2, and 3, including FM-2 Wildcats, F6F Hellcats and TBM Avengers, strafed, bombed, torpedoed, rocketed, depth-charged, fired at least one .38 caliber handgun and made numerous "dry" runs at the Japanese force when the American planes ultimately ran out of ammunition.[3][4]

Sprague's task unit lost two escort carriers, two destroyers, a destroyer escort and several aircraft. Over a thousand Americans died, comparable to the combined losses of American men and ships at the better known Battles of the Coral Sea and Midway. But in exchange for the heavy losses for such a small force, they sank or disabled three Japanese cruisers and caused enough confusion to persuade the Japanese commander, Vice Admiral Takeo Kurita, to regroup and ultimately withdraw, rather than advancing to sink troop and supply ships at Leyte Gulf. In the combined Battle of Leyte Gulf, 10,000 Japanese sailors and 3,000 Americans died. Although the battleship Yamato and the remaining force returned to Japan, the battles marked the final defeat of the Imperial Japanese Navy, as the ships remained in port for most of the rest of the war and ceased to be an effective naval force.[5]