Hands down.... PB Blaster is first choice.
For the ones that bust anyway, Kroil is the penetratingest stuff ever.
I have about 4 cans of PB around the shop and one can of Kroil. I use them all. Be sure to let the rusted up bolts soak a while before busting them loose. PB has a catylist that eats the rust but it takes a little time.