i use four knots in total:
i *really* like the marlin spike hitch on my hammock toggles and tarp & hammock guylines. so much faster/easier to tie/untie than anything else.
i use the grapple hitch for the tarp ridge line around the tree, or to secure anything to a line. it is a very secure knot, it's adjustable (can slide) and it's easy to untie even on zing-it (but not in cold weather and gloves - i'm waiting for some tarp flyz for that ;-)
prusiks and zeppelin bends are the two other knots i use.
i love them all, so useful! :-)