My votes in no particular order...

Bally Old Chicago, Hang Glider
GTB: Jumping Jack/JITB
Jungle Queen/Jungle Princess
Target Alpha/Canada Dry
WMS: OXO, Solids N Stripes

Old Chicago is a special game... when I had mine, I set it to a tough score and 3 ball play... loved peppering the add bonus spinner and then hammering the center hole... very difficult to control the ball with the placement of the 10 point bumpers, but that's important to beating this game with any regularity, and adds so much to the challenge of the game. If her play surface was in better shape, I'd have kept her

I had a Jet Spin not too long back - IMO it gets boring just hammering away at the roto or trying to reset the roto target value... cool artwork and the extra ball target placement is cool, but all in all, I think if you're a decent flipper control guy, the game is just too easy and you'll lose interest.
My faves are:

Bally: Old Chicago
GTB: Joker Poker, Royal Flush, Jungle Queen
WMS: OXO

Damn, forgot about Night Rider... was looking at CrazyLevi's post... not sure what draws me to that game so much, it's crazy good; there's always one at either A-town or York, and I tend to be drawn to that game a LOT.