I use the smallest stuff I can get. What I have now is about 1/2 as thick as mechanical pencil lead. I use it for everything... I just got spoiled with it I guess... melts super fast, easy to work with, and I get it by lb from eBay.
I'm pretty sure it was .020, and it was either a 5 or 8lb spool... but I can't find it on there now. I need to start keeping my eyes open for it now because I'm starting to run a little lowish. But, it was a massive spool and it was $18... I remember that.
Good stuff.