"Best" is subjective. Most soft solder will work OK.
I use rosin core electronics solder. I tin the joint on both sides with plumbers acid flux paste and steel wool. I then clean off the flux, it is very corrosive. I then coat the joint with liquid rosin flux. Then jig it. Then heat to melt the solder.
I infer that if you use only acid flux paste that the joint will eventually rust and the joint will fail.
I am taking a que from how shotgun barrels and ribs are soldered. I solder all of my under ribs this way and have never has one loosen over time or fail the ring test after soldering.