Simply put. M-18.
Yes, we've joked around about it for what seems like forever, but now that we have a new Heavy German Tank, we need something to counter it.
The Nashorn is a good idea, but it wasn't specifically designed to counter the threat of Hitler's panzer divisions in WWII.
The M-18 was almost 35 mph faster (according to Wiki), 25-30 mph (according to various other sources). The armor of the M-18 was almost equal to that of the Nashorn (almost).
The Nashorn has it beat with the 88mm cannon, but the M-18 could be fitted with the 90mm or 105mm variant. (I have looked up site after site, I couldn't find any with any info about those variants or if they were even used in the war, although I know prototypes were built). The 76mm cannon, iirc can destroy a King Tiger. 115mm penetration at 1k is more than enough to enter a King Tiger at various areas (without factoring in deflection).
This is just a sort of spitballing thread, if you have any info on the cannon variants please feel free to discuss.
I posted the stats on the M-18, Its 25mph cross country, so don't expect it to be anything fancy, M-18 was fitted with a few varities however the 76mm is all you will have, with the same ammunition as the M4A3(76) we have.
M10-
Crew 5
Combat weight 32.6 Tons
Max Speed cross country 20mph, Road 30mph
Armament: 3-inch gun M7 in Mount M5, .50 cal M2
Main gun ammunition: 54 rounds, 1,000 rounds .50cal
M62 APC muzzle velocity 2,600ft/sec penetration of 93mm at 500 yards @30 degrees
M93 HVAP muzzle velocity 3,400 ft/sec, pentration of 157mm at 500 yards @30 degrees
Armor: 57mm gun mantlet, 25mm Turret sides, 38mm UFH, 50mm LFH, 19mm upper hull side, 25mm lower hull side
M-18 specifications:
Crew: 5
Combat weight 20 tons
Max speed: cross country 25mph, road 50mph
Armament: M1A1 76mm in M62 combination mount + one .50cal
Muzzle Velocity: 2,600ft (M62 APC) 3,400ft (m93 HVAP)
Penetration: @500 yards @ 30 degrees 93mm (M62) 157mm (m93)
Armor: 13mm front, 13mm side, 13mm rear, 8mm top, 6mm floor
M-36 would be the choice, since we would get the M-10 and the M-36.