I don't think there really is a "must have" of anything. Individual preferences are so varied across the forum. I think you can pick any one light and easily find 10 who love it and 10 more who dislike it... even the most popular ones.
I think the one "must have" that everyone can agree on is that it must possess some degree of durability and reliability.
That's it!
A collection can't ever cover everything, if it does, you have a museum, not a collection anymore.
So, you have to find out what lights you like first and then decide for yourself. As time goes by, the "must have" lights change, unless you decide to collect all the milestone lights, but then you're gonna pay more for 10 historical Lumen then you would pay now for 700 actual Lumen.
Btw, when I got my first lights, I didn't think at all about a collection, I just wanted some bright and modern lights. Now, after about 30 more lights, it's still the same, I buy a light because I think I really need especially that one. So, I can tell you about the actual "must have" : EagleTac T20C2!
A few weeks before, it was the NEX GDP ss bezel (will be one of my favorites), the SR3 (shouldn't have bought it) and the RRT-2 (my best thrower so far). Before those I can't really remember... :shakehead