Personally, I think the oil rig is one of the best. I went there and came back with so much military gear.
My 2nd nomination is West Isle airfield. It spawns helis on the helipads, and a tank can spawn inside the blast door hangar.
3rd nomination is Bunker behind south woods. Just plain ol good loot.
4th nomination is Rose Port. It's amazing how much metal and construction gear yo can find there. Also, there is a Military tent inside the exit tunnel of rose port and the hidden room, if you can enter it, also has military loot and medical loot.