Most combat hardened ship in the U.S. in December 1941?

USS Constitution, of course!

This was easy since I am reading a book about that ship currently.

After much excitement I'm sure here is the one I was thinking of:

HMS Warspite is the one on my mind. By December of 41 she had fought enough in WWII to wear out another set of guns. She had them replaced, some AA upgraded and damage repaired at Bremerton Ship Yard near Seattle.

She arrived in August of 41 and departed at the end of the year.

en.wikipedia.org
HMS Warspite (03) - Wikipedia
en.wikipedia.org en.wikipedia.org

Some of them sailing ships had pretty storied careers also and I don't want to argue against them or their legacies in any way.

Skipping details of collisions with other ships, bombardments, and inadvertent target practice some pre Pearl Harbor Warpsite highlights include:

- The WWI battle of Jutland where she scored her first hit on the Van Der Tann. Later Warspite lost steering control to a hit and circled HMS Warrior drawing the attention of the Germans off the over matched cruiser and onto Warspite albeit unintentionally.

- After the WWII German invasion of Norway Warspite and escorting destroyers were sent into the tight water of the Narvik Fjord to clear a force of German destroyers. Besides the action destroying the destroyer flotilla, Warspite's Swordfish float plane became the first to sink a U-Boat in the 2nd world war.

- In the Mediterranean at the Battle of Calsbria Warspite scored the longest hit on a moving battleship in history, a 26k yard hit on the Italian battleship Giulio Cesare which caused enough damage the Italians disengaged.

- She was part of a screening force for the Taranto attack where bi-planes sunk some Italian battleships in harbor a year before Pearl Harbor.

- Warspite was part of the force which sunk three heavy cruisers in a night action and turned back an Italian fleet again at Matapan.

- During the evacuation from Crete which helped necessitate the Bremerton repairs, Warspite helped fight off the Axis air force and received damage from a 500lb bomb presumably dropped by an Axis pilot who would claim 110 air to air kills in the war.

OHHHHHHH physically in the U.S., not in the US Navy. Gotta pay attention!