I've always just used a paste flux and solid core lead/tin solder. Both for the initial tinning of the surfaces and then the actual soldering after positioning and clamping everything together.
Never had a complaint.
Most any flux that doesn't seem to work well is often because the operator over heats everything and burns the stuff. That destroys the protection from oxidation the metal surface gets from it during the soldering,
Get the surfaces clean, well fit, parts evenly tinned and most any decent flux will give you good results if you don't over heat things.
