Time Pilot 84 is great.
I used to have a Future Spy. It's a board swap (and 1 switch swap & joystick up/down swap too?) for Zaxxon, so it's easy to have a 2-in-1 cabinet with Future Spy. Oh, and I suppose Super Zaxxon is an easy board swap too...
Future Spy's decent--not great, but decent.
-Jason