What was the most effective tank destroyer of World War 2?

'Effective' will certainly depend on what you mean by 'effective'. In either case, the German Panzer IV/70 (V) was one of the lowest tank destroyers (a height of just 2m., including periscopes, or 1.85m. without periscopes) and it had the firepower of the Panther.

Its main deficiency was the long gun, which stressed the frontal roadwheels, however this was somewhat solved by the implementation of steel tyred roadwheels.

The best US tank destroyer was the m36 with a 90mm gun on a m4 chassis. The best German one was the jagdpanther with a 88mm gun mounted on a mark v panther chassis. The best Russian one was the su85 with a 85mm gun mounted on a t34 chassis. Of the 3, only the m36 had a moveable turret. The others had to have the whole tank moved to aim the gun.

Out of all the research that I have done on Russian and German tanks, I would say that the SU-100 was the best tank destroyer. It too was on the T-34 chassis, but it had a 100mm gun rather than an 85mm gun. The only problem was the actual weight of the gun itself.