The one that's easy to sand and is easy to apply. If you ask which one, you will get 100 answers. For just crack filling between planks, I simply used Titebond II glue mixed with sawdust from the same piece of wood as the planks, and sanded the extra off after drying. For filling in low spots in the first layer of hull planking, just about any wood filler will do.
The solution is simple: make some from scrap wood. I have a couple plastic container with cherry, walnut, and pin sawdust saved from previous jobs on the bandsaw. I've gotten years of use filling in cracks and gaps with them.