I think the 3" Harken ratchet blocks works great. I have a smaller ratchet on one of my fish and it doesn't work as well. I have heard that one of the new Ronstan ratchet blocks works very well also.
I have seen several different ones. I and many others use the Harken model HO19. It has a 10:1 holding strength and can be turned off to spin freely with a simple slide switch on the side. A spring under it hold it upright. $53.47 from APS. Makes a big difference.