While the VC Firefly with the 17pdr is a great tank, I believe if we are going with a Sherman we should consider the M4A3E2-76 Jumbo Sherman. While the Easy Eight is a great tank, I think the much heavy frontal and side armor of the Jumbo would be much more useful.
The Comet and Challanger, along with the Centurion would make great British tanks, all will the 17pdr. Btw they did carry HE rounds.
As far as Russian tanks go...I'd love to see a KV-1C or KV-85 tank, only problem is no plane or the PzIV would be able to kill it with the exception of a rear shot at close range. The Tiger's 88 barely ever made a frontal shot kill on it, and Panthers did only fairly better. A T-34/85 would be a very good choice given it's gun and speed.
Back to US tanks... an M-18 has a top speed of about 55mph

the 80 is in km/h. It's kinda hard to say what it's impact would be. It's opened topped so even a .303 or 7.9mm gun has a chance of destroying it, and it only carried 15 HE shells at the max (It was a TD unit after all)
A M-36 Jackson would give a good trade off of excellent gun and speed to again, weak armor and an open top, but it's 90mm is a much more flexiable gun compaired to the M-18's 76mm.
The Panzer IVH is by no means a slouch of a tank, from the lessons learned of the PIV F2 and the earlier IV tanks, and was quite the late war slugger. (Albiet more of a Close support then the Panther or Tiger) and that has to be taken into consideration when developing an allied tank.
Below are most of the stats for the tanks mentioned so far...take a looksie and see what you think would offer the most while keeping the balance

- Jig
M4A3E2-76 "Jumbo" Sherman
Speed 35.4 km/h
Off Road 12.0 km/h
Rev Speed 18.0 km/h
Body Mass 37.0 tons
Front Armor 140.0 mm
Side Armor 75.0 mm
Rear Armor 40.0 mm
Front Turret 167.0 mm
Side Turret 153.0 mm
Rear Turret 152.0 mm
Top Armor 25.0 mm
Turret
Traverse Speed 24 degrees per second
Elevate Min -10.0 degrees
Elevate Max 25.0 degrees
Projectile Velocity = 792ms
Projectile Mass 7.0kg
Caliber 76mm
AP Ammo 30
HE Ammo 31
KV-1S (Lighter armor, yet much faster variant)
Speed 45.0 km/h
Off Road 12.0 km/h
Rev Speed 22.5 km/h
Body Mass 42.5 tons
Front Armor 100.0 mm
Side Armor 60.0 mm
Rear Armor 50.0 mm
Front Turret 109.0 mm
Side Turret 85.0 mm
Rear Turret 95.0 mm
Top Armor 30.0 mm
Turret
Traverse Speed 22 degrees per second
Elevate Min -3.0 degrees
Elevate Max 30.0 degrees
Velocity 655 m/s
Projectile Mass 6.3kg
Caliber 76mm
AP Ammo 50
HE Ammo 64
British Comet
Speed 46.7 km/h
Off Road 13.0 km/h
Rev Speed 23.0 km/h
Body Mass 35.8 tons
Front Armor 101.0 mm
Side Armor 50.0 mm
Rear Armor 50.0 mm
Front Turret 101.0 mm
Side Turret 50.0 mm
Rear Turret 50.0 mm
Top Armor 14.0 mm
Turret
Traverse Speed 24 degrees per second
Elevate Min -12.0 degrees
Elevate Max 20.0 degrees
Projectile Velocity 792 m/s
Projectile Mass 7.7kg
Caliber 77mm
AP Ammo 39
HE Ammo 19
Sherman M4A3E8 "Easy Eight"
Speed 41.8 km/h
Off Road 16.0 km/h
Rev Speed 21.0 km/h
Body Mass 32.0 tons
Front Armor 94.0 mm
Side Armor 38.0 mm
Rear Armor 40.0 mm
Front Turret 89.0 mm
Side Turret 65.0 mm
Rear Turret 64.0 mm
Top Armor 25.0 mm
Turrent
Traverse Speed 24 degrees per second
Elevate Min -10.0 degrees
Elevate Max 25.0 degrees
Projectile Velocity 792 m/s
Projectile Mass 7.0kg
Caliber 76mm
AP Ammo 30
HE Ammo 31
M-36 "Jackson" TD
Speed 48.3 km/h
Off Road 14.5 km/h
Rev Speed 24.0 km/h
Body Mass 28.1 tons
Front Armor 88.0 mm
Side Armor 25.0 mm
Rear Armor 22.0 mm
Front Turret 76.0 mm
Side Turret 32.0 mm
Rear Turret 25.0 mm
Top Armor 15.0 mm
Turret
Traverse Speed 30 degrees per second
Elevate Min -10.0 degrees
Elevate Max 20.0 degrees
Projectile Velocity 853 m/s
Projectile Mass 10.9kg
Caliber 90mm
AP Ammo 37
HE Ammo 10
Sherman IIC "FireFly"
Speed 36.0 km/h
Off Road = 12.0 km/h
Rev Speed = 18.0 km/h
Body Mass 34.0 tons
Front Armor 69.0 mm
Side Armor 25.0 mm
Rear Armor 25.0 mm
Front Turret 80.0 mm
Side Turret 26.0 mm
Rear Turret 25.0 mm
Top Armor 19.0 mm
Turret
Traverse Speed 24 degrees per second
Elevate Min -10.0 degrees
Elevate Max 15.0 degrees
Projectile Velocity 883 m/s
Projectile Mass 7.7kg
Caliber 77mm
AP Ammo 50
HE Ammo 27
T-34-85
Speed 55.0 km/h
Off Road 16.5 km/h
Rev Speed 27.5 km/h
Body Mass 32.0 tons
Front Armor 90.0 mm
Side Armor 52.0 mm
Rear Armor 56.0 mm
Front Turret 129.0 mm
Side Turret 103.0 mm
Rear Turret 76.0 mm
Top Armor 20.0 mm
Turret
Traverse Speed 17 degrees per second
Elevate Min -3.0 degrees
Elevate Max 30.0 degrees
Projectile Velocity 792 m/s
Projectile Mass 9.02kg
Caliber 85mm
AP Ammo 35
HE Ammo 20