Depends. If one might be particularly complementary to my group, I might let that one go last. It might not be the best looking or nicest, but it might simply mesh best with what I have. Other times, I might keep the best one for myself. There have also been times where I held onto the "worst" (relatively) for myself only for it to end up going from ugly duckling to swan during a phase of growth while the "best" one ended up less impressive later on. Lots of sellers have experienced sales of ho-hums that developed into stunners and stunners that ended up ho-hums. Sometimes one has a personality I prefer most and that one might be held until the rest have gone. I prefer my project males to be low in aggression while high in eagerness to breed and finding that combination is not always easy for some species, so a gentle romantic male is going to be favored by me for holdback status over a knock-around male. Also, if an animal has more of an interactive personality well beyond expectation of a food reward, then I might keep that one back or save it for last. If I have some price differentiation within a group and am not listing them all at once and am instead listing them in series, I typically list the least expensive animals first instead of last in a season so that my earlier customers are somewhat less likely to have hurt feelings over pricing. If person one pays $800, person two pays $900, and person three pays $1000 for clutchmates that have only mildly nuanced differences, people are more likely to be at peace with their purchases than if I went in the reverse order (where a price-sensitive customer might feel like he/she "overpaid" by $200 if he/she bought the $1000 animal out of the gate when the $800 animal became available later). This is not perfect. Tastes vary and disposable incomes do, too, so someone could be upset that the slightly better animal was one they were willing to pay for when they bought the decent one earlier, but I am only one person and cannot always accommodate that sort of thing, so I pick a path as I described and proceed.
Then there are extensions of "worst" in a group that could need greater address. If I have an animal with obvious issues (issues I can readily detect; maybe a limp, a funky-growing bite pattern, or maybe slower than average growth relative to its clutchmates), I might be willing to sell it with said issues declared (typically with a discounted price relative to its clutchmates). If the issues are more severe (to me) than that, I hold it back and do not sell it. I might offer it up for adoption, I might euthanize if the quality of life does not pass muster in extreme cases, or I might do some sort of special work on the animal if I feel that such work might develop the animal from a unsalable state into one worthy of being sold with confidence. It really comes down to the specific situation.
This is tangential, but there are also people who request that I find various things for them and secure those for them. I rarely do that, but...sometimes. It usually means I have to eat a financial risk on the promise to pay. Some people are reliable for following through. Others not. For those who are reliable, I might have to buy multiples of something to secure the one or few that they would eventually select and be happy with. The unchosen animals (that simply did not strike a particular and subjective fancy) then go into the scenarios described in paragraph one of this post.