To the best of my knowledge...
In an M8
Vs a Panzer IV:
Front: Front of the turret will disable the turret in one hit (aim for the middle) at a decent range (~1000) yards.
Side: I haven't had much luck... firing at the back of the side will sometimes disable the engine, if you have already knocked out the turret you'll usually get an assist when some tank finishes off the guy.
Back: Back of the turret will knock out the turret, back of the body will knock out the engine (inside of about 500 yards, which is the only back shots I've ever taken)
Vs a Tiger
The Tiger is immune to the M8, you can't damage it from any side at any distance
Vs a T-34
I haven't fought a T-34 in a M8, so I can't say. Odds are good the M8 won't do much to it.
In a T-34
Vs a Panzer IV: Hitting the front of the turret will disable the turret at 800-1200 yards. Just aim at the middle.
Hitting one in the side of the body will usually kill one in one hit from 800 yards in, hitting the side of the turret usually ricochets off.
Hitting the back of the turret killed the tank in one hit the only time I've done it.
Vs a Tiger
The Tiger is basically immune to the T-34.. in the DA some guy let me sit 5 yards behind him and it took about 10 shots to knock the turret out (hitting the rear turret), the rounds ricochet off every other surface at every range.
Vs a T-34
Haven't done this much.. but the T-34 appears pretty much immune to other T-34s. I tracked one and then backed up when I drove up next to one headed towards me (came over a hill and there he was). I used the coax gun to track him, the HE round I fired there had no effect. All his rounds (all both of them) ricocheted off the front armor of my tank)
In a Panzer IV
Vs a Panzer IV
Again, hitting the front of the turret will knock out the gun, usually hitting the front by the driver will kill the tank (the little vision slit, aim there)
Aiming at the front of the side will usually kill the tank, aiming at the back of the side will kill the engine.
From the back, hitting the rear turret will disable the turret, hitting the body will kill the engine (and the tank, usually) .
Vs a Tiger
I'm sure someone better in tanks can tell you this.. I've killed them before by hitting them by the driver from the front, and you can disable their turret by hitting it, but it is by no means a sure deal.
Vs a T-34
Side shots work very well from 1200-1600 yards, usually first shot will kill the turret and second shot will kill the tank.
Rear shots again from very long ranges will kill a T-34 in one hit (in my experience, very long range is ~1600 yards)
From the front, it is hit and miss. Most time if I get hit in the turret from the front at 800-1000 yards I lose the turret. Longer ranges than that and it usually ricochets off. I had one Panzer bushwhack me as I killed his buddy, he came up over a hill and fired on me frontally from about 100 yards and it richocheted off.. so there isn't a guarantee.
I tend to lose my turret very easily in the T-34. but then survive a pounding that would make someone playing over a rusty can & string connection proud (5-6 hits for no damage, till I drive away).
Best advice I can give from the T-34 perspective is aim for the turret first no matter which angle you are firing from, it seems the easiest part to get through.
Any tank Vs an M8..
Hit it anywhere. Hell, use HE rounds, then you don't even have to hit it, just get close.