What are the hardest members of k-pop groups to collect?

now this was originally a post aimed towards twice specifically, recently i’ve been thinking about picking up more members i only collect nayeon as of now.. i know i wanna add tzuyu but im not so sure?

so i’m asking what members from kpop groups are hard to collect? i know many of my collecting friends say chae (from twice) is hard??? and i collect jikook from bts also nothing hurts more than seeing a jungkook pc for almost $20…