Of course, must have is definitely an opinion. I can tell you what I have on mine that I really enjoy:

Mirror blades, flipper toppers, shooter protector,
Pinstadiums with Xeno Mounts,
Pinwoofer sub , tweeters, and power supply
Lior Art of Pinball bullet shooter rod and custom surround. Lior action button surround
Back box and under cabinet lighting (my son daveyvandy added these himself)
Titan sling and flipper rubbers
Pinmonk coil fans (these are almost must have)
JJP topper (honestly not that great)- Lior is working on a high end one
Lots of PEG clear washers on posts to prevent playfield pooling- may no longer be an issue…
Custom metal drum set that my son made
GNR banner and pinball machine cover blanket.

Good luck and enjoy the game!

Lower posts by the slings and upper playfield post. I'm not sure if this is still a problem with the latest run of these games. The plastic washers seem to work wonderfully and have no effect on my (mediocre) play.