Interesting question.
I'm not sure it is possible to suggest just one aircraft.
May be top 5 list as the following one, in no particular order:
1. I-16 (all types serviceable) - because it helped VVS fighting force to survive until more advanced types were adopted.
2. Pe-2 (again, all types) - as it was the most numerous short range bomber and the only dive bomber through all the war. (Just few squadrons implemented dive bomber technique, but still...). Also probably the most effective recon plane until Tu-2 was introduced in that role.
3. La-5F/FN - since it bridged the gap in speed and climb between VVS and LW fighters. That resulted in the boost of confidence of pilots and in creation of new tactical methods. Appearance of La-5 also brought Alexander Yakovlev (who was not just the designer but successful apparatchik and one of main decision makers in aviation industry and the favourite of Stalin) back to his senses and made him to work hard to create aircraft better then Yak-1/7.
4. P-39 (all types) - due to its somewhat unique role in VVS history. Ace maker, trend setter in several areas: heavy cannon, B&Z attack, extensive radio comm usage, new tactics.
5. Il-2 - because it was the most numerous CAS aircraft through all war on the Eastern Front. Successes of this type were probably exaggerated by propaganda and too often Il-2 squadrons were used as expendables... But Il-2s were everywhere, they harassed the enemy constantly, they helped infantrymen to survive and to move forward, etc.
Honoralbe mention:
1. P-40 - for being a gap stopper in certain areas in critical periods.
2. Li-2/PS-84 (licensed C-47) - transport (remember quality of Soviet road network...), night bomber, special missions.
3. Early Yak family - another "gap stopper" and better (compared to other types available) escort fighter which saved lives of many bomber and sturmovik crews.
4. B-25 (probably A-20 as well). - Reliability, protection, range, equipment - all that was much better then what Soviet industry could offer. Safety and comfort for crews and opportunity for engineers and designers to learn a lot of new things - and to implement them in domestic designs later.
5. Yak-9 family. - Many modifications, most numerous fighter type at the end of the war, the only long range fighter (Yak-9D/DD).
6. Last but not the least: three B-29s which landed safely in USSR territory. Test flights and studies of those machines has prepared historical decision to launch Tu-4 project which resurrected heavy bomber force after the war and "revolutionised" USSR air industry.
Note: I did not mention Yak-3, La-7, Tu-2 - they were very good and modern for their time, etc. But they arrived in large numbers after "the turn of the tide" ( to borrow from another thread).