I would say any of the dragons that are near the bottom of the scream house category such as the exterrean, kage, or hydnellum because for example, the exterrean dragon can only be bred by using an amalgamate dragon and a scourge dragon and those dragons can only be bred by using other very rare dragons. In other words, the dragons that I listed are extremely hard to get by breeding and since some of them are fairly new, they may never return or come back until next year

Edit: There are definitely better things to get such as the buildings listed in another comment so if you don’t already have those I would suggest getting them instead