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.