Ummm... DacronŽ, mostly, for utility cord - probably better than half the time. The rest of the time I use 550 cord (nylon). Everything else gets used - just not as frequently, and those are the cords I always recover, chain, and re-use. I use nylon mason's twine for some things, accessory cord for a few things, and (if I get it inexpensively) sisal or manila for a few things. I really prefer very much to not use cotton cordage if I can avoid it.
Generally, nylon holds knots rather well, actually - perhaps you are using a small diameter braided nylon cord? Those can be a PITA at times - slippery with some knots and difficult to untie with others. Try some paracord and some polyester (DacronŽ) cordage.
HTH
Tom