The soviets started the war (in my opinion) with some of the best tank guns available in mass production, the 76.2mm guns such as the T-34s F-34 and KV-1 ZIS-5 were among the best anti tank guns carried by a tank at the times, when you consider what rest of the world had in the form of 75mm low velocity guns (which mostly never fired Armour piercing ammunition until later on) on French CHAR b1 and the M3. However as the war went on, dew to the conditions the Soviets were in, implementing radical new technologies, especially new caliber guns with new ammunition was not as easy as many like to think, when they had to manage between resource and labor they had after the invasion in 1941 which took the country in a shock. In war time, and especially under the harsh conditions the soviets went through prior to the invasion, it was really hard to implement new and bold equipment. For example, the T-34s excellant 76.2mm gun did not get replaced until the 1944 with the 85mm (various calibers from 51 to 54 caliber barrels i think. There was a very long barreled hyper velocity flat trajectory 57mm gun developed for the T-34 to make it into a better anti tank platform, but however the STAVKA cancelled it probably because implementing such a gun could potentially delay production of existing tanks not to mention all the efforts of testing it and setting up new logistic chains etc.
The 85mm gun on the T-34/85 was a very powerful gun, but many say its potential was not exploited due to poor ammunition. Again, this maybe because the STAVKA choose to mass manufacture millions of less sophisticated "cruder" ammo than say invest on few thousands of really fancy Sub caliber HVAP rounds (which soviets actually implemented though in few numbers later in the war. Soviet anti tank rounds in general had tendancy to shatter on impact, possibly dew to quite conservative design based on older designs of anti tank penetrators not able to keep up to date with advances in modern rolled homogeneous armour, usually caused by high stresses in the penetrator.
The IS-2 tank was also considered to fit the excellent 100mm D-10 gun from the su-100 which had better ballistic performance than the D-25T 122mm gun on IS-2, but was droped in favour of the potential problem with logistics and offsetting production (as new machine tools had to be devised to manufacture 100mm guns and its ammunition, when it was far easier to use the machines used to manufacture the A-19 gun that has been produced for quite a long time.
Also, i wonder how you guys rate soviet tank guns with German ones? Germans probably had the best ballistic performance guns in the war, like the Pak 43 l-71 which wouldnt have looked out of place in early cold war post war tanks with its unmatched accuracy and effective range and penetration. However, these guns were very "single purpose", they were only useful against armoured targets and possessed relitivly little HE frag power from its relativity puny 88mm shell if one compares them to the slower 122mm D-25 guns on IS-2 which were devastating against fortifications. Even in anti tank role, the heavy 25kg shells of IS-2 can seriously damage a tank without even penetrating its armor, and cause epic spalling inside. The americans and british also developed pinpoint accurate high velocity anti tank guns such as the american 76mm on later shermans and the british 17 pdr "firefly" gun. As i see it, soviets mainly focused on a versatile main gun balancing HE anti personal/fortification capability with decent anti tank capability than an anti tank bias. This could be possibly due to operational doctorin, as soviet tanks attacked mostly in the war, whereas Germans after kursk were in a retreat and naturally were on a defensive position, usually concealed firing at moving soviet tanks coming from the horizon.
Does anyone know how good were late war soviet ammunition? I read somewhere that an improved BR-471(b variant?) could reliably penetrate panthers front armour at 2500meters!! If true, that is a phenomenal performance and is quite different from the popular myth that the Is-2s gun is some sort of a lazy inaccurate "derp" howitzer, when infact its accuracy was not far off to the TIGER i l-56 88mm gun (even though the muzzle velocity is much lower).