One thing no one has mentioned yet is rope. Can't have too much in my opinion. It would be ideal if one type and size would work for every application but, unfortunately, on my boat at least, this is not the case. The size of the halyards is limited by the maximum size of the masthead sheaves to 3/8 inch rope. While plenty strong enough for any application except possibly anchoring and mooring lines, that is just too small to be comfortable in the hands for sheets and downhauls etc. so I wind up with 3/8" halyards and sheets of 7/16" braided polyester (Dacron), as are the spare jib halyard and the topping lift which can double as a spare main halyard. Besides the lines rigged, we carry an extra 100 feet of 3/8" and 200 feet of 7/16" braided Dacron and 200 feet of 1/4 inch Amsteel(Dyneema) for emergency standing rigging repairs. We also carry a spool of 3/16 polyester for lashings and other miscellaneous uses around the boat.
Anchor rodes and dock lines are of 1/2 inch three-strand nylon. We also carry a thirty foot pendant with a nylon thimble and shackle on one end and an eye splice on the other plus four fifty foot lengths and one additional heavy anchor rode, 300 feet, all of 5/8" three strand nylon. That is in addition to 100 feet of 3/8 inch proof coil chain plus two fifty foot lengths of 1/4 inch chain for the 1/2 inch anchor rodes. (We carry two 10kg Bruces and two Danforths)
I'm sure some of you are laughing and shaking your heads but I do this for a reason. During our unexpectedly long Pacific crossing, our halyards chafed nearly through at the masthead. I had to end for end the sheets due to chafe and replace the jib tack downhaul with an old sheet which nearly chafed through by the time we reached land. As for the ground tackle and mooring lines; I've been through two hurricanes. The morning the sirens go off is no time to discover you don't have enough rope to secure the boat.
Ideally, I'd have everything rigged with 1/2 inch braided dacron and buy a 600 foot spool of the stuff. But that would require major modifications to the masthead to accomodate the sheaves and all the blocks on the boat would have to be replaced. 3/8 inch would be strong enough for everything except anchor lines and would fit all of the blocks and sheaves but it's just to small for my hands so as long as I have this boat I guess I'm stuck with at least three kinds of rope for rigging.
Lest some "Yachtsman" get the urge to correct me on calling it rope. I learned as a trainee on a square rigger that it's rope until you put it to work. A coil in the locker is always rope until you reeve it through a block or attach it to the clew of the jib. Then it becomes a line.