The tank that benefits the most of add-on armour

... is the Panzer IVH.

My stats with it are atrocious because I used it for warbond and SUMMER tasks and capturing instead of for tank killing first, but it's actually BY FAR the best of the Panzer IV series.

It's also quite good-looking (for a tank) with all that add-on armour.

Those 18 mm track segments aren't anywhere as good as 18 mm RHA,

but the armour thickness is at least nominally close to 100 mm in most places

of the front. Angle this tank by 20° a bit and it's about as tough as a Tiger H1

facing its opponent at a 0° angle.

Historically, this tank was the last quality Panzer IV series. The J series was a low quality design with less rubber, lesser steel quality, no powered turret traverse (hand crank only!). The H series was a real tank, J series was mere cannon fodder, mostly tasked to secure the flanks of Panthers - to die instead of them, since a flanked Panther is as unable to survive as a Panzer(kampfwagen) IV.

Similarly, Panzer IIIN (the III series with 75 mm L/24) were often used to protect the Tigers in the heavy tank detachments and to relieve them of the dreaded scouting duties.