Does Anyone Have Ideas For Must Have Coins?

IMHO, another question needs to precede this one .... namely, what constitutes a must-have coin? Arguably, the answer will differ from collector to collector. Is the must-have coin one that ....

1) is needed to fill an album hole?
2) is desired for its historical provenance?
3) is atop the condition census?
4) is a new variety?
5) is one that few collectors will ever own?
6) is one that tells of an historic event?
7) is any combination of the above .... or?

I recently acquired a coin that's been a must-have piece on my want list for over 25 years now. I jumped at the chance to own it and paid far stronger than 99.9% of collectors would. I bought into the history of the piece, of which around ten are known. Lacking knowledge of the history, I might only have paid $5-$10 bucks for the piece. The knowledge, the knowing, is what should drive collectors to determine what their must-have pieces are, methinks. AMEN ....