When arguing against subjectivism (i.e. desire-based accounts of normative reasons) Parfit puts a lot of weight on the theory's failure to accommodate the datum that everyone has reason to avoid agony: agony-indifferent agents are making a mistake, according to Parfit, whether or not they would admit this upon reflection. I find this pretty persuasive, but I think the argument is more ambitious than it really needs to be, as in its current form it would seem to target nihilists as well as subjectivists. A more modest, and hence even more persuasive, argument against subjectivism would instead focus on the positive normative claims entailed by subjectivism.
For example, imagine Bob is wondering whether or not to torture a puppy. He knows all the relevant facts, which include the following: the puppy would suffer extreme pain, Bob himself would feel nauseous and subsequently loathe himself for so acting, and all else is equal. I take the following to be an obvious truth: it would not be contrary to reason for Bob to refrain from torturing the puppy.
But desire-based views cannot secure this datum. For we may suppose that, no matter his feelings about the matter, Bob has an iron-willed determination to torture puppies. Upon reflection, he endorses torture as an end in itself, as indeed his supreme goal (or desire) that overrides all others, including his future happiness. Desire-based theories therefore imply that Bob ought to torture the puppy, and has decisive reason not to refrain from doing so.
This result is clearly absurd. One might reasonably be a nihilist, and deny that there are any normative properties at all. But if you're going to play the normative game, you cannot plausibly hold that what Bob really ought to do in this case is to torture the puppy (ruining his own life in the process). His iron-willed determination to follow through on his inexplicable and perverse desire is not properly described as the epitomy of reason and good sense. If normatively assessable at all, it is surely just insane.