Halo 1 was the best of the FPS.
Halo 2 was okish, it was very difficult to get past 40 by yourself because of modders, and hacking was its downfall.
Halo 3 is just terrible, and they are trying far too hard to milk out the series.
Halo Wars was actually very fun, but only if you like RTS. It was actually the original game engine for Halo, but bungie didn't like it and starting moving the game into their own direction. After halo 1, the creator of Halo said screw this and bailed off the truck with his own game engine, which is now Halo Wars.
So any true halo fan will love that too