I spend most of my time in a GV but I rarely up anything but an M18. I've discovered a few sweet spots on most every tank
Panzer F and G: From the front, hit them in the turret. Very rarely takes more that one to the turret. From the side, Shoot them in the back half of the side armor. Again, very rarely take more than one toejam.
Panther: From the front, hit the very bottom of the hull as low as you can without shooting under them. It takes 2-3 shots depending on the range. From the side, go for the front half of the side armor. One shot will do it almost everytime.
Tiger: From the side, put your shot in the front half of the side armor. Depending on range it will take 1-2 hits for the kill.
Tiger 2: The beast! From the side, shoot anywhere from the front of the side armor to the middle. Takes at least 3 shots and occasionally more. From the rear put one in the turret to disable it, then put 1-2 shots in the rear hull.
T34/85 and 76: Much like the Panther. From the front, place your rounds as low as possible on the hull. You can also put your first round in the turret to disable it, but it needs to be pointing at you or at a 90 degree angle to you, else it will ricochet. From the side, place it near the center of the side armor but a little to the front.
All M4's: Again much like the Panther. from the front, your rounds should hit the hull as low as possible for a one shot kill. From the side, place it near the front of the side armor. Usually only takes 1 round but occasionally 2.
M8's: Pretty much anywhere will one shot kill them.
M18: I personally avoid shooting these in the turret. It very rarely results in a one shot kill. Instead go for anywhere on the hull or sides no matter which way it's facing. I've never had it take more than one shot at any range to kill one when I hit somewhere besides the turret.
M3's: From the front, put your round right in the grill. From the side, avoid hitting high and to the rear, instead hit the near the front, perferably in front of the driver in the engine.
I believe I covered everything. I intentionally left out jeeps and SKD's (for the simple fact that I can't remember ever seeing or killing an SKD.)
If anyone has anything to add to this list feel free.