No such thing as a "Must Have", IMHO, except for a person who needs a defensive arm.
But as far as collecting... one of the issues about getting caught up in that attitude is that it's really easy to end up with a soulless cookie-cutter gun that everyone has. Granted, there's a reason everyone has one, right? Consensus is a powerful indicator of quality or satisfaction. But sometimes it's best march to the beat of your own drum (when speaking collection/shooting, not function based purchasing, of course). Of course, if you're buying based on function your "Must Have" is what you "Must Do" (defend, hunt, compete, etc).
For me, I have more handguns than I'll ever need but I certainly wanted each and every one of them. My long guns are completely informed by what I "Must Do"... and sadly, in my state, what I "Can Do" (otherwise, I'd have suppressed arms- completely illegal in this state).