The metabolically most active plasma lipid is:

Plasma lipids consist of triacylglycerols (16%), phospholipids (30%), cholesterol (14%), and  cholesteryl esters (36%) and a much smaller fraction of unesterified long-chain fatty acids (free fatty acids or FFA) (4%). This latter fraction, the FFA, is metabolically the most active of the plasma lipids. HARPERS ILLUSTRATED BIOCHEMISTRY30th ed, Page No:254