knowing what your application is would help a lot, but i usually avoid soldering unless it involves directly attaching stuff to a PCB. for example, have you considered terminal blocks? more reliable and nicer to work with imho.
but if you want to solder...
use a eutectic alloy because it melts at the lowest possible temperature, and it melts at a single temperature instead of over a temp range. as BukitCase said, for a Sn-Pb alloy, that's 63/37. forget 60/40
as cvairwerks said, tin your wires first before soldering them together.
better still, put a bit of flux on them first. i use this stuff, but there are many good choices:
https://www.amazon.com/gp/product/B00425FUW2/?tag=atomicindus08-20
and for a clean look, finish it off with heat shrink tubing.
ok and finally for your actual question, what's a good solder? many good options, but for general purpose stuff, just go with kester Sn-Pb 63/37 in your desired diameter.
you can read about kester's various products here
https://www.kester.com/products/category/solder-wire/leadcontent/57
the last thing you need to decide on is the flux type.
for general purpose, just go with
kester 44. this has a RA flux core that is fairly aggressive and great at wetting.
if you are working on pretty clean, shiny stuff that doesn't need as aggressive of a flux core, consider
kester 285 (RMA flux).
you can read a little about different flux types here
https://evertiq.com/news/16783
and
https://en.wikipedia.org/wiki/Flux_(metallurgy)#Rosin_fluxes
here is a decent thread:
https://www.eevblog.com/forum/beginners/which-kester-solder-to-buy/
what % flux?
for kester 44, imo just get 2.2% flux. since this RA flux is already aggressive, 3.3% isn't needed, and you can always add more flux separately
putting it all together, my overall recommendation for you is kester 44 63/37 2.2% in a reasonable diameter (0.02-0.03").
if you can find it, just buy kester 2463370017
this is kester 44, Sn-Pb 63/37, 2.2% flux, 0.025" diameter
that's what my next spool will be.
gl, hope that helps
