For cycling, Swiftwick Pursuit are my hands-down favorite (tbh, they're my favorite for non-cycling, too - I'll wear them any time). Dense and cozy cushion under the heel and forefoot, but thinner and breathable elsewhere. My second choice would be Smartwools in Light Elite - similar to Swiftwick Pursuit, but not quite as nice, though they can often be had at deeper discount than you'll generally find for Swiftwick.