Ping pong balls always go down well with Max, nice and cheap too which is a bonus when he bats them out of sight and I have to open yet another packet
Anything with catnip is normally pretty good!
Pets at Home do mice with microchips in them, they squeak when they're batted, so really good interactive toys which don't need human intervention, so great when you're out at work.
The best by far has to be Undercover Mouse which I bought Max for Christmas. I'll see if I can find my old thread with some pics - its fabulous