The Spit 8 is probably the easiest to get kills with. I find that anything short of an 8 kill sortie with the Spit 8 is a little light.
Good guns, good speed, turns very well, climbs very well and is quite forgiving.
The only thing that sucks about it is the roll rate. A bit of rudder helps, though.
Motherland is right about about picking a plane for its history and sticking with it, though. It's more rewarding that way.
I love 109s and fly them most of the time. I love everything about that aircraft.