With the T28 you’re looking at one of the strongest guns on one of the slowest vehicles in the game. You should always try to outmaneuver the T28 or expect heavy damage on your part.
As I said before, the front is a thing to avoid, but if you see it and manage to get a shot off, you should aim for the weakspot right below the gun or the driver to the right.
The backs two thirds of the T28 are quite vulnerable with the mid third representing the ammo rack and the back third the engine.
Like most other tank destroyers, the back is what you should really go for. Hitting the engine will reduce the T28’s turning rate and therefore render it almost immobile.
The ammo rack is located in the middle above and below the top side of the tracks on both sides.