I wouldn’t do an airforce or naval base because modern air and sea systems couldn’t be repaired and resupplied in 1941. Tanks still run on diesel and can fire the munitions used in WW2 so that’s a possibility but they still can’t be repaired and transporting them to Europe would be a significant undertaking. Imagine bringing them to the past only to lose half of them to a uboat in the middle of the Atlantic. They also wouldn’t be very useful in the Pacific.
I’m going to go with Liberty. It has army special forces command, a Ranger regiment, and a modern medical facility. I think the most valuable thing we could send back to 1941 is knowledge and training. Deploy the Rangers as trainers and advisers, send the medical personnel to teach modern trauma medicine and lead surgical units.
Missions to secure nuclear weapons happened when the Soviet Union collapsed and the Russian government invited the US in to do some of them. The missions wouldn’t be to take the sites and disarm the weapons, the goal would be to make sure all weapons are accounted for, being properly maintained, and under the control of a government. It would be to make sure the Russian arsenal wasn’t being sold or taken by splinter groups or extremists rather than to disarm Russia. I expect China would be doing a lot more of it than the US if Russia collapsed in the near future but the US as well as UN nuclear inspectors would almost certainly have a role.