The M26 Pershing is just a more expensive (and worse) American version of the Tiger I; losing overrun and -0/-1/-0 vs vehicles for 2 points more and +2/+2/+1 vs infantry. For 3 points more I find the IS-2 much better in standard games, as the extended range doesn’t matter and 7/7 defence makes it immune to most regular infantry, and even some AT infantry have trouble with it. While the IS-2 has -1/-1/-0 AI, it gains +1/+1/+0 AT.
I’ve found the MGC has a good attack and is fast, but is very vulnerable; which is about what a US Tank Destroyer should be. I don’t use it, but I don’t think it’s terrible.