If it’s not the cost then silver solder is the strongest and easiest
option. You can copper plate the solder line with old pickle and iron
wire (there is a recipe for a copper plating paste in the Practical
Recipes eBook http://www.ganoksin.com/gnkurl/l0 ).
If the join is a large or complex one then using soft solder is a
good option and you can copper plate this too.
But if you don’t want to plate for some reason then you could use low
silver brazing rod, you can get this from most fabricating
engineering supplier. The colour of the join will be brass though.
Phosphor bronze brazing rod, which is copper coloured, could be used
but its pretty tricky to use by all accounts (I’ve never used it).