OP

Quoted from NicoVolta:

Good observations here! Particularly from Pecos who raised some interesting contrasts.
And now, let's overthink the solution...
Flip flags - Flip Flop versus Wizard! Good call here. Both are 4-players, both are awesome late-period Ballys, and both have distinguishing characteristics of their own. Flip Flop has the lower pops and bounce-back outlanes... whereas Wizard! has cool "flag powerups", a spinner, and of course "the" pinball theme which was "the" game Bally announced to the world that, indeed, they were taking over.
I think Wizard! has the edge if we pit these two against one another.
STARS is a strong contender. Not a pure EM, but a fantastic hybrid with real chimes and great rules. Also the only classic Stern here to represent the company which is a factor.
Aztec vs. Argosy... another good call. Both late-period Williams 4-players, both fast and furious, but even though I haven't played Argosy as much it does seem to have the edge in long-term enjoyment. Aztec's basic premise is "destroy the spinner" which is easy to light. The rest is largely secondary. So I'd have to tilt Argosy here.
Jumping Jack vs. Captain Card. Another good call here! Both are drop target feasts. Captain Card has a fabulous fan of resetting drops and huge scoring (doesn't it go to a million?) whereas Jumping Jack is a multiplayer with a great twin lower pop arrangement. In the keeper pile we have an Old Chicago which is the all-time #1 twin lower pop game... but we don't have a Gottlieb. And since we dropped Flip Flop the lower pops aren't an overweighted feature. Finally, since multiplayers are more fun when playing with others, JJ gets the nod over the solitary wedge. JJ has better artwork too.
Hang Glider vs. Little Chief. Both with a similar layout and a "build up and cash out" strategy. Hang Glider has a spinner and drop targets, but do they add much to the gameplay? Doesn't look like it. Also the top mini-buttons are more novelty-esque than functional. I also admire Little Chief for being an evolution of Kordek's relatively terrible Big Ben into something great with only a few tweaks... and for that it makes Hang Glider relatively superfluous.
Mini Pool is rare and fun, but like its twin Target Pool it is similar but inferior to 2001. So it isn't essential.
Dixieland... looks like it is a must. The most innovative 1960's Bally single player, the only zipper-flipper here, the longest outlane ever, and lots of creative diverters which add to the unpredictability. It's weird and different and just too cool not to be included.
Which leaves only one to pick from:
Big Day
Bow & Arrow
Pioneer
Snow Derby
Solids & Stripes
Big Day is rare and looks cool but the gameplay resembles an alternate version of Gator/Sheba/Campus Queen. I wonder if Sheba (despite its rarity) should be swapped for this one? Hmmmmm. Will have to get it running and play it to be sure. A contender.
Bow & Arrow. Beautiful condition. Cleared playfield. Strong contender.
Pioneer. Hmmmmm. Great artwork and good multiplayer with "double double" bonus which is fun to snag. But... overall it is a bit slow and rollover-ish which isn't my favorite style. Perhaps it should go to the museum.
Snow Derby. Winner of TPF 2016 best 1970's EM and has been cleared, rebuilt, polished, and custom painted. It looks and plays flat-out amazing. Probably the best Snow Derby in the world. But truth be told, it's not among my personal favorites. Would part with it for the right price, but for now can keep the visitors at the museum entertained.
Solids & Stripes. Damn. Another one of Norm Clark's wacky genre-busting games. Has to be played as a 2-player game to shine, but so does Surf Side so that's not a dealbreaker. I wonder if I'd pull one out of the "keeper pile" to make room for it...? Hmmmmm.
2001 - cannot remove
Flipper Fair - cannot remove (it's personal... had it since I was six... and it is fun!)
Freedom prototype - no freaking way
Joker Poker - cannot remove (SS version)
Little Chief - cannot remove (it's the Hang Glider replacement)
Old Chicago - abso-freaking-not
Out Of Sight - possible
Queen's Castle - need to play it, but rare and beautiful and only Zaccaria here so probably not
Rancho - possible but it is a real beauty
Sheba - possible if replaced with Big Day
Sinbad - cannot remove (SS version)
Skyrocket - no way jose
Spanish Eyes - cannot remove
Surf Side - cannot remove
Thus... would I trade Solids & Stripes for...
Out Of Sight
Rancho
Probably not. But crap... I just bought a 1977 Big Deal. What about that one? Hmmmmmm. Nah.
So the sell list would likely be...
Aztec
Big Day or Sheba
Big Deal
Captain Card
Flip Flop
Hang Glider
Mini Pool
Pioneer
Snow Derby
Solids & Stripes... I'll try to fit it in somehow. Nothing else quite like it.