This is difficult, there are lots of great games still to be deisred. But for now my top 5 would be:
Nr. 1: Moon Cresta

I played it a lot, back in the eighties and I think that the game is great. I´m not sure if the shown cabinet is an original dedicated one, but I would trade almost any one of my present games just to get it. The game is so great!! Fortunately I have the original PCB, so I play it now and then in a generic cab, or in a Candy (yeah, Moon Cresta on a 29" screen looks awesome).
Nr. 2: Tempest

Like Moon Cresta, Tempest is a great game. I played it recently for the first time in it´s dedicated shell, during South Coast Slam. Unfortunately the game broke down preey quick, else I would have played it the whole weekend. What I love about the game is the frentic gameplay, the sounds and of course the vector visuals (yeah, I´m a vector man

). As with Moon Cresta, I would trade almost any present game that I own for it, even Gravitar. You don´t happen to have a Tempest, André?
Nr. 3: Gyruss
I like Gyruss, he is a very kind guy, and ..... Hm, wrong sidetrack.

Of course I mean the game:

Again, great game, with great stereo sound and stunning visuals. The Cabinet is great also. I don´t think that I will ever get a Moon Cresta cab, and the Tempest will also be very hard to get, but Gyruss should be doable. And in the meantime I play the original PCB of it.
Nr. 4. Frogger

What is there to say, very fun and addictive gameplay and I like the looks of the woodgrain cabinet. I´m hoping that this game will also end up in my collection one day.
Nr. 5: Juno First.
For pictures, see this topic. This seems to be a popular game, which is highly desriabel but hard to get. As Tempest I played this game for the first time at the South Coast Slam and I loved every minute of it. Would be great to add to my other games.
This is my top 5 for now, and there is little chance that it will change. But as I sais, there are lots of other great games, like R-Type, BurgerTime, etc.