I've long put the British 17pdr, German 7.5cm L/70, German 8.8cm L/56, and US 90mm M3 guns in roughly the same category for penetration. Obviously, the British 20pdr, German 8.8cm L/71, and American long 90mm (T15-series, for example) would be superior to the shorter guns. So, where do the Soviet 100mm guns fit? Well, for me, somewhere in the middle, but leaning towards the latter group, especially in the postwar years.
Again, though, you have to realize a bunch of things. First of all, comparative tests from the period are terribly flawed, to say nothing of the raw data from the various nations. What counts as a "penetration", what was the Brinell rating of the target armor, etc. That means that most of this is guess work at some point. Also, ammunition quality matters quite a lot, generally US and USSR ammunition in WWII was pretty terrible but got better as the war went on. German and British AP ammunition tended to be of a consistently higher quality than US and Soviet ammunition during much of the war years. There are plenty of exceptions to these, though. For example, Oskar Schindler was famous for producing thousands of rounds for Germany that were of intentionally poor quality.
The Panther's 75mm and the British 17pdr and frighteningly similar guns when you compare them, much long the American M3 90mm and the Tiger I's 88mm gun are very similar to each other. The 7.5cm KwK 42 L/70 and QF 17pdr are notable for being the lowest caliber guns in the group by quite a fair margin. They were higher velocity, incredibly accurate, and had amazing penetration. They were also smaller, lighter rounds, so you could reload them faster and carry more of them. The downside? The HE rounds were pathetic and their barrel life was quite short. This isn't just an interesting factoid from the testing grounds, these all translated to actual battlefield trade offs.
The American long 90mm guns were just experimental and never had all the bugs worked out. The postwar guns actually relied heavily on HEAT rounds, so the AP performance was not as important. From what I've read, M48 Pattons with 90mm guns could reliably knock out T-54/55s in combat, but struggled against IS-3s. The fantastic British 105mm L7 did not have that issue and was better in every conceivable way (comparable to the Soviet 115mm).
My personal rule of thumb is that, other than a few outliers like the 57mm ZiS-2, a Soviet gun is generally best compared to the Western gun of slightly lower caliber. That's usually do to a lower velocity and results in being able to carry fewer rounds, being slightly more inaccurate, and taking longer to load. It has the advantage of a shorter development cycle, extended barrel life, and greater HE content.
Everything's a trade-off.
Cheers,
Logan