Personally, Spy vs Spy 2 was my fave, but most likely because I'd played it before the first one, which I think most people would say is the best, and certainly most challenging. I remember when I first played the 3rd one, I pretty much instantly dismissed it.
I'm not sure if it was because it lacked the atmosphere of the others, or because it just felt like more of the same. However, I played it again yesterday with an open mind and it's actually quite good fun.
It's a shame it was a bit of a commercial flop, as apparently there was a 4th one planed which was going to be in space. This makes sense as in the 3rd one, when you complete the level, you get in a rocket and blast vertically out of the screen.
What's your thoughts on this game or any of the others in the series?