This is a simple answer, thankfully.

60/40 Sn/Pb. Don't use rosin cored stuff. Use flux on the holes before you start soldering; make sure that when you've completed the job you rinse the board thoroughly to get rid of the flux - it's acidic and may damage a board over time.

Don't use the Leadless RoHS garbage, it's the very devil to work with without melting your face off, and it may not be as reliable as good ol' 60/40.

Good luck, happy hacking! :approve: