I'll just have to be a voice of dissent here and say that I feel that the Yak-3, Yak-9U and La-7 were among the finest combat aircraft produced in the Second World War, with some qualifications.
First of all, they are primarily low to medium altitude fighters. Above about 5000-6000m they were outclassed by their Allied and German opponents, with the possible exception of the Yak-9U, which remains competitive up to about 7000m. Below 3000m the Yak-3 and the La-7 are clearly superlative; they combining excellent speed, climb and some of the best manouervrability of any WW2 combat aircraft.
Secondly they were all relatively short ranged, so none of them were capable strategic fighters. The fundamental character of the Soviet air efort in WW2 was tactical; short ranged, predominantly in support of dedicated ground attack aircraft and medium bombers. So in a western front situation they are not particularly useful.
Thirdly, there were many variations in the build quality of Soviet aircraft. So some aircraft may be well behind on performance, depending on the factory complex they were produced in. However, when you look at the variations in performance for Allied fighters during official testing, this is perhaps not suprising. The A&AEE noted that a 6-7% variation in speeds were not unusual for factory fresh airframes according to their testing. I believe Curtiss also had some build quality issues during the war.
Forthly, Soviet aircraft technology was still behind in terms of basic comfort and convinences for the pilots. Soviet aircraft didn't tend to have all the bells and whistles, moderniation and automation present in German or Western Allied aircraft. Soviet aircraft tended to be relatively more uncomfortable for their pilots, with poor ducting, heating and fume extraction. Perhaps a small point, but still something that counts against them. This plauged some Allied aircraft as well though. The P-38 was notorious for is bad heating system. Typhoon pilots were forced to fly with oxygen massks on at all altitudes because of the possibility of carbon dioxide gasses escaping in to the cockpit.
Fifth, Soviet armament was excellent, but it had very limited ammo duration and was relatively light compared to that installed on Allied and Aixs fighters. Twin 20mms or 2 HMGs and a single 20mm, all nose mounted, are more than enough to threaten any fighter or light/medium bomber, which is all that Soviet fighters were likely to encounter on their front. However, they dont have the depth of fire time of US fighters. But US aircraft are very much an exception in these terms, generally hauling more weight of ammunition than fighters from any other nation during the war. RAF research showed that a fighter pilot was only likely to expend around 85 rounds of 20mm ammo on any sortie, less than half their total load out. Longer ammo duration is nice to have, but the Soviet reasoning was that extra armament and ammo was useless if a fighter couldn't get behind its opponent. Therefore they went for lighter more nimble fighters, with less firepower, than for heavier fighters with more armament (a la FW-190A, P-47, F4U).
Sixth, Soviet fighter pilot training was of highly variable quality depending o the unit and front they were serving at. Some Soviet pilots recieved 200-300 hours training before going into combat, some recieved as little as 20-30 and were expected to 'learn on the job' as it were. The general standard of German and Allied aviatiors, at least until 1944, were higher then their Soviet counterparts. After this Allied training contiued to improve, while German pilot qualit entered a steady decline.
When you look at the generally established performance of Soviet aircraft in terms of speed, turn, ceiling, climb ect you can see that the late war Soviet fighters are all highly competitive dogfigthers. Not necessarily superior, but competitive. They are, without exception, small, nimble, lightlweight interceptors capable of sucessfully taking on anything thrown against them, if the conditions or right. Below 5000m they come out well against any opponent in WW2, Allied or Axis.