Harken ratchet blocks have been the leader in this field since the begining.  Tough reliable, they just work.  Schaefer snatch blocks are the simplest and most reliable but for those who like one hand operation check out the Harken.  For large boat application you may want to check out the Antal snatch blocks.  Both Schaefer and Harken make good wire blocks.   Go to http://www.riggingandhardware.com/c-732-blocks.aspx for a complete listing and current pricing on these products.