WOZ, Tron, and IM. However, IM is still too new to me to even judge for something like this.
WOZ is just a bitch. It went through a rough patch with me where I almost sold it. But then the software just got better and better until it achieved greatness. And then after that, they still made it even better. It reached the point that playing it is always fun. I had 4 red letters flashing in Rescue last night, and drained trying to get up to the castle playfield. The layers just keep going, and there are still things I haven't done in the game after more than a year of trying.
Tron. What more can I say about Tron? I've made it to Poral, but it was pretty short. I've struggled to get back there, but it is always just out of reach. The game always seems to have the right amount of flow, speed, and difficultly. I don't fire it up for a quick game, I fire it up for a war.
IM. I've only had my VE for a little while. So really it is too new to judge. But that game is CrAzY. I have never seen a game that delivers so much satisfaction and frustration so quickly. Sometimes I hit the start button more than the flippers. I already know the sound when Tony says "Kill Power" followed by the start sound. While playing I keep telling myself "Last game, time to go", but then another game starts. And when you get the game rocking, it is just amazing. So yeah, it's staying.