If we're also include the LL stuff, the Spitfire IX was probably the best (with P-39 second best) until P-47D was delivered (with P-63 as second best).
To be honest Tomo I have not read enough on the subject of lend-lease aircraft to Russia. Although I have read quite a bit.
Spitfires over the Kuban and several threads (semi-official discussion).
I am still in the very middle of researching the lend-lease aircraft sent to Russia, so I do not have all the answers at this time. I am actually gearing up to read:
http://ww2aircraft.net/forum/aircraft-pictures/russian-47-a-4805.html.
From what I have read so far the Russians praised the Spitfire L.F.IX mostly for its higher altitude performance (imagine that). I have also read that most if not all Mk.IXs supplied to Russia were second hand.
I do not know for a fact at this time if all the Spitfire L.F.IXs were second hand or not. Just for a little perspective check I decided to do a side by side. The following figures are for vehicles in excellent condition. I have used Spitfire L.F.IX B.S.543 with a Merlin 66 and a P-63A-8 tests for comparison.
Note: The P-63A-7 was the first version of Kingcobra supplied to Russia, so the figures should be fairly close.
Spitfire L.F.IX ------------P-63A-8
Altitude / Speed / Climb.
Meters / MPH / FPM
SL.--- 336 / 4620--------378 / 4900
1,000 350 / 4653--------393 / 4775
2,000 365 / 4695--------405 / 4575
3,000 380 / 4302--------413 / 4325
4,000 384 / 3922--------420 / 3970
5,000 385 / 3860--------424 / 3520
6,000 398 / 3608--------423 / 2925
The Russians in WW2 and I fail to see how the Spitfire L.F.IX supplied to the VVS could be considered completely superior to the P-63A, especially at the altitudes that most combat was conducted. The Russians seemed to be partial to centerline armament also.