I like Cardas Quad Eutectic. It flows smoothly and gives a good solid joint. There's a seller on Ebay that sells ten and twenty foot lengths for a reasonable price if you don't want to buy a lb. roll.
http://cgi.ebay.com/CARDAS-Quad-Eutectic-Silver-Solder-10-Free-Ship-US_W0QQitemZ130227548529QQcmdZViewItem?hash=item130227548529&_trkparms=72%3A552%7C39%3A1%7C65%3A12&_trksid=p3286.c0.m14
http://cgi.ebay.com/CARDAS-Quad-Eutectic-Silver-Solder-10-Free-Ship-US_W0QQitemZ130227548529QQcmdZViewItem?hash=item130227548529&_trkparms=72%3A552%7C39%3A1%7C65%3A12&_trksid=p3286.c0.m14