Grigoriy Rechkalov
From Wiki.
Says the majority of his kills were achieved on P 39N-0 number 42-8747 and P 39Q-15 number 44-2547.
http://en.wikipedia.org/wiki/Grigoriy_Rechkalov
Listed as 61 in a P-39Q here, but tally varies according to sources.
http://www.fighteracesonline.com/grigori_rechkalov.htm
Rechkalov was an outstanding fighter pilot, a natural shot who had personal conflicts with his division commanders. It was said that Rechkalov was more interested in building his personal tally than being a fighterleader. Nethertheless there is no misunderstanding of his ability's in the air, flying his P-39Q Air cobra Rechkalov scored 61 victories in just 122 combat missions. Rechkalov was awarded the HSU twice, he became a Major-General of Aviation in 1951.
But listed at 50 here.
http://acepilots.com/planes/soviet_p39_airacobra.html
And listed as 44 here, but it has the most detailed accounting of his claims.
http://surfcity.kund.dalnet.se/soviet_rechkalov.htm
If the last one is correct, then he falls behind Aleksandr I. Porkryshkin who is listed with 48 claims.
Here, link also provided above.
http://acepilots.com/planes/soviet_p39_airacobra.html
Not clear, but perhaps 55 in mostly in 1943, all in P-39s, according to Wiki.
http://en.wikipedia.org/wiki/Alexander_Pokryshkin
He flew these P-39s, P-39K-1 BuNo 42-4421 "White 13", P-39N BuNo 42-9004, "White 100", "white 17", 41-38520, and P-39Q-15 "white 50".
An article of his service here, doesn't detail his claims completely, though.
http://mig3.sovietwarplanes.com/mig3/pokryshkin.html
Best wishes,
Grant