The absolute best and easiest place to aim (as far as actual being-shot-at usage goes) is the side of a Panzer, in the middle and just above the track. Fire from a perpendicular angle, and you can penetrate right to the ammo and destroy the tank outright. My record is three shots into that region for an explodey catastrophic kill.
You can also fire at the side and towards the rear to hit the engine. This one takes more shots for a catastrophic kill; maybe five, maybe ten. Firing into the turret-hull seam also works, but can be unreliable. Have gotten catastrophic kills with anywhere from four to ten shots into that region.
I haven't had much luck firing into the rear of a tank (hard to see any landmarks on the hull to aim at), plus these are only likely to be mobility kills, which is not very helpful if you need to get rid of it right now.
There are some critical spots at the front of a tank, but I also have not had much luck there (firing into the viewport never did much for me, and neither did firing into the hull MG gimbal mount, behind which there is apparently an ammo store).
But now we get to the important part: the best way to disable a tank with an AT rifle.
On a server that has aimbot hullgunners disabled, then this is quite simple: Your priority is to disable the turret traverse, and then to disable the engine. Completely destroying a tank is not desirable because it means two things: a) The tank respawns a few seconds later, and b) You are now five or ten cartridges poorer, and must slink off to resupply.
But if you simply disable vital components of the vehicle without causing so much damage that the commander can scuttle it, then the tank does nothing but sit there and look pretty. You kill the turret traverse, and the cannon and coaxial MG have a very narrow field of fire. You kill the engine or the tracks, and they won't even be able to maneuver to use the hull MG. If your team understands this, then they can leave those tanks unharmed and steamroll the enemy, who is now without close support.
So TL;DR: approach tank perpendicularly from the side, aim at the turret-hull seam until the turret no longer turns, and then put two or three into the hull towards the rear of the vehicle to destroy the engine. Then tell your team not to even touch the tank over VOIP.
Oh, if you can, team up with some friendly AT riflemen and lay an ambush. I did this once on Red October with two other team mates: this Panzer was ripping our team up and we could hear it making its way to one of our caps. We set up on different sides of an open square and listened to it inch closer. The Panzer lumbers into view, we wait until it's in the clear, everyone starts firing in unison, and the tank is in flames less than two seconds later.