Looks like Tempest is on everyone's list.
I was feeling pretty good that I actually *own* a Tempest game until I went to the California arcade thing last month. Somebody had brought in a Tempest where they had switched in some custom ROMs to get new level shapes. Some of them were inside out ! Excellent. Now I'm jealous again.

OK, my dream list.
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Tempest
Robotron
Qix
Quantum (slightly rare, but quite cool)
DeathRace 2000 (silly fun, extremely rare, haven't seen one in decades, strangely was considered "too violent" way back in its day)
Defender/Stargate

For Pinballs:
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Hyperball (30 or 40 pinballs in flight at once, crazy madness! Useful for waking up the children or annoying the neighbors.)
Black Hole
Gorgar
Then probably FunHouse or Black Knight
Finally I'd probably include "Invasion from Mars" if it weren't so enormous and so likely to be an utter maintenance nightmare.

Bonus wishlist item: An old Atari Football would be nice for any rec room (Black and white. The X's vs. the O's. Giant trackball that one spins wildly.)

Then, if I still had just a little more room needing to be filled, I'd get a pinball game called, I believe, "Orbit." It's not that fun to play, but it's really fun to watch someone else play it. The playsurface isn't flat but is some kind of parabolic clear plastic. So the ball orbits around the flippers and just generally does weird bizarre things.