60/40 rosin core.
Something like this would do:
http://www.fort777.co.za/index.php?main_page=product_info&cPath=813_848_854&products_id=8626
The thickness is a function of what you're soldering. You want the solder to melt quickly (thinner melts faster) but you also want to product enough solder to cover the joint (thinner wire produces less solder).
1mm (edit: its actually 0.7mm) thick suites me fine for my solder stuff (pots, patch cables etc).
Hope that helps.
