Hm. Very hard call here. Protector: Reclaiming the Throne, Zwingo, Mastermind: World Conqueror, and Balloon Invasion all come to mind...
If I had to pick one, I thin I'd choose either of the Protector games. (Protector and Protector: Reclaiming the Throne) Either one. The one that wasn't picked would be my number two anyway. XD
But for the record, Zwingo, Balloon Invasion, and Mastermind: World Conqueror would also be in my top five as well.