Any good solder as long as it is not meant for plumbing. I use this stuff:
https://www.amazon.com/Solder-Sn99-3.../dp/B072WP4H99
It is a lead free solder so it is safer than the older stuff. It is mostly tin, hence why it is said to "tin" a wire. It is easy to work with, almost too thin. Never have had a problem with it.