The 128/55 is a poor choice. Seperate ammunition slows the firing rate. It also had problems in terms of weight of round and was nothing special in accuracy and reliability.
The German 70 caliber guns (both the 75 and 88) suffer from poor (relatively speaking)accuracy due to barrel vibration and short barrel life. Both also suffer from poor recoil systems that take up alot of space. Sure, both had good penetration but at a fairly high cost in other desirable properties.
Personally, I would choose the Soviet 100/60. This is a gun with excellent penetration, good reliability and, reasonable weight. It was successful enough that it is still in service today. In this sense it out lived the US 90mm which was only an average performer as far as penetration goes.
The Soviet 85 and 76 have mediocre penetration performance for their size and neither is a particularly outstanding gun while the 122 suffers from the same problems as the German 128; it's just too big to be useful.
The British 17pdr also suffers from excessive barrel wear and has poor accuracy compared to the US 76mm for example. It's short post war life demonstrates this quite clearly.
So, my vote goes to the Soviet 100/60.

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