You kind of answered your own question. If you want the complete experience, well off course you`d buy the complete set, wouldn`t you as that be the complete experience, right?

For me, I chose what I liked the sound of and would increase my pleasure of the game. for instance I like making up my ships so I got the Builder`s Kit.

I like interesting random events, so got the Mega events; not much happens but when it does it`s interesting.

I like the idea of finding new interesting stiuff so got the Lost Treasures.

And I liked idea of paying Mercs to do jobs for me when I quickly needed units so got the Mercenary pack. I will probably get the rest soon except the Snaithi as they seem silly.

But what I like may not be what you like. But if you want the complete experience, then buy all of them.

My faves are Mercs, Precursor Worlds, and Lost Treasures.  I have (and like) them all.

Because I play mostly sandbox, my personal picks would be:

- Precursor worlds

- Lost Treasures

-Mercenaries

I think the scenario ones are really cool, though, so if you like that kind of content than it is easy to recommend those as well. Not many 4x games appear to do that type of content.