My opinion is it's hard to define one of them as the "best" as you can be successful in any plane is you fly it enough. If we're talking about just the plane then I'd say it's pretty close between all the 109's, 190A5, 190D9, and Ta-152.
I think the 109F4, followed closely by the 109G2, is the most well balanced of all the 109's. It combines the 109's excellent climb rate, excellent rudder control, great flaps, and just overall fighting ability that all 109's have, but it balances it far better than any other 109. The later in the 109 series you go, the heavier they get, the less nimble they become, and they start to focus more on speed and climb rate. The 109F4 just balances everything about the 109 so well and it's so light and nimble you can really throw it around the skies at will. The 109G2 is a close second for most balanced 109. It's got a more powerful engine than the F4 but it is also a bit heavier and not as nimble as the F4. The K4 is a fantastic plane, but the gun is just troublesome. You have to be a REALLY good shot to get hits with that 30mm. You can out fly your opponent all you want, but if you can't hit your shots then you're never going to get a kill. That's the main reason why I think the K4 is the worst of the 109 series. If the gun wasn't so bad, or if we could put the MG151/20 in it instead, then the K4 would easily be one of the best IMO.
The 190A5 is kind of the 109F4 of the 190 series. It's light, nimble, and balances the 190's characteristics quite well. The 190D isn't as nimble as the A5 but it's fast! Speed is a good survivability trait to have and the Dora has it. It's also a pretty good turn fighter if you fight in the vertical and use it's great climb rate. The Ta-152 is just a beast. It's a rocket with a prop! Not only is it fast, but it can turn very well at most speeds! The 152 is plagued with being too much of a beast and can be difficult to handle, but if you master the plane then there is very little in the skies you'd have to worry about.
My list of top 5 German planes IMO:
1. 109F4 - I can fight my way out of any situation in this plane
2. 109G2 - Same as 109F
3. 190A5 - Amazing fighter that can BnZ and TnB, the latter of which surprises lots of people
4. 190D9 - Very smooth machine, it's speed and climb rate allows you to quickly work your opponent down until you kill him
5. Ta152 - FAST! You can rope people so easily in this plane, and if you're forced to turn fight it can do so!
I would put the 109K4 on this list if it wasn't for the gun. The plane is amazing! The gun is troublesome.
