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I like 3/8" dia. I think that is the largest recommended dia. for a 27 foot vessel for any application. Smaller line can bite into your hand if you really have to tug on it. Heavier line feels better but can be to heavy on the clew in light winds. I like the look and feel of a single braid, (no outer cover), very soft, flexible and medium stretch. The Sta-Set line is more durable though and more often the choice. (New England Rope, West Marine Catalog)
Is there an absolute best? Idunno.