The #1 must play machine out there is Pinball Circus, without a doubt. Not because it's a great game (it isn't), but due to it's super uniqueness and rarity (only 2 in existence, I think is the claim), you just simply have to try it out. I found it quite fun, myself, but also quite easy. Not a game I would keep coming back to, but I'm glad to say I've played the only one out there. Where is the other one, I wonder?
Beyond that, the PHOF just has lots of stuff you've probably never seen or have only seen in photos, unless you tour the country regularly attending pinball conventions. Lots of EM games, too, if that floats your boat. I quite enjoyed playing Q-Bert's Quest, Fireball, '79 Star Trek and many of the woodrails, EM's and others that just don't make it to my neck of the woods out at the Texas Pinball Festival.
And while it's probably cheaper to just charge a flat fee and play any game for free after that, like they do at pinball conventions, I kinda like the fact that it's pay-to-play there. Makes earning a free game (or matching) mean something again...