If we're talking Pro only, Iron Man probably would be my choice. Very high tension game, and at the same time you can use it as a great training machine.

If we're allowing the LE/Premium, XMen is an easy choice for me. XMen without the magnetic-spinner-thing though seems kinda silly, would be like tron without the disk.

But I really enjoy XMen LE - In a lot of ways I feel like XMen is Tron-only-better, feels like he fixed a bunch of problems with the layout (Gem was nigh impossible to hit; rogue is reasonable, jets on tron are dangerous enough to make the outer loop a shot you avoid; xmen jets are safe, arcade scoop not backhandable; blackbird scoop is, nothing good to shoot on tron from left flipper most of the time;xmen right flipper has xavier and wolverine (plus disk/magneto, orbit-through-jets, and a scoop that usually isnt worth shooting, which are present on both)), added some interesting rules and, maybe best of all, brought back the traditional extra-ball-lit-after-completing-n-modes.