I only fish bamboo rods, ranging from 4-wt through 7-wt, and I've never settled on a particular brand or style of line. I sometimes use a silk line (and I do dress it with Otter Butter as narcodog recommended), but I mostly use synthetic lines. In general, Cortland 444 in double taper works well on most of the rods, and the Hook and Hackle lines that you can pick up at a bargain also perform well in DT. In response to Dluver's question, I have used Cortland Sylk, and I find it to underline the rod a bit (matching the 5-wt line with a 5-wt rod). Sylk is a smaller diameter (like actual silk) than typical synthetic lines, and that may seem like an important consideration due to small guides on very old rods, but I find that my old rods with smaller guides such as the Phillipson Pacemaker and Heddon Black Beauty shoot line just fine with standard fly lines.
It all just comes down to matching the rod and the line with your individual casting stroke. Keep trying different lines until you get one that really sings for you, and don't be afraid to go a line size heavier and a size lighter when you're trying them.