I used to fly Spit XVI's exclusively and I'd say it's the best all around fighter in the game but not the best all-around plane.
It's really got a great balance of speed, manouverability, firepower, climb rate, dive rate, roll rate and acceleration allowing it to perform in any and every style of fighting. Among those tools there is something there to defeat any opposing aircraft. Also among those traits there is something that will allow any opposing aircraft to beat the XVI if the XVI pilot's not aware of his opponents strengths.
The only thing that keeps the XVI from being the best overall aircraft is it's lack of ord loadout and that moves it into a pure fighter catagory IMHO. Due to that the F6F-5 is the best overall plane in the game followed closely by the P-38G but that's just my opinion.