IS-3 is fully upgraded.
I'm 2 vehicles away from Charioteer, RU-251, and Pershing, which I'm just working towards simultaneously with the majority of emphasis on Charioteer.
Unrelated to this, but I'm also two vehicles from E-100, and one away from the S-51.
Has there been consideration of adding the T26E4 to the mix? In a frontal dog fight using APCR, there isn't much that can beat it.