I like the high tensile one made by Crosby. Hamilton marine has them search for "HI TEST SLVR PIN". http://store.hamiltonmarine.com/browse.cfm/shackles/2,1034.html

They're much stronger than other shackles for a given diameter, and (for example) you can use a 3/8" shackle with a 5/16" high-tensile G40 chain, and the SWL will match.