I would list at least these three in the top 5 (in no particular order at this point):

The Marder in all its variants. These vehicles were widespread, easy to construct, made best use of scarce resources for the Germans and proved on the whole very effective. When Marders mount ex-Soviet 76.2mm guns appeared in North Africa the British were at first stunned and alarmed that the Germans had finally found a way to put an 88 on a vehicle and field it in large numbers.
In the East these vehicles filled a critical gap for the Germans giving them an effective mobile antittank gun in 1942 that remained viable right to the end of the war.

The M 18. America's solution to the problem. Incredibly fast. In fact still one of the fastest, if not the fastest, armored vehicles ever built. The M 18 was also relatively small and had an excellent gun, sight and, control system that allowed it to get on target very rapidly. Coupled with the Self-propelled tank destroyer battalion's reconnissance section these tanks could appear and disappear all-too-quickly against an enemy armored attack.

The StuG III/IV makes the list by virtue of its ubiquity in German service. The early versions (A - F7) with the short 75 are hardly "tank destroyers" in any sense of the word. The late versions proved effective in this role more because these vehicles often had decent crews than because they were really top notch tank killing systems. The late war addition of a commander's cupola was a tact admission that better crew visibility was necessary if this vehicle was to be a tank killer rather than a mobile artillery piece.

Some that shouldn't be included:

The Hetzer. A horrible improvisation. This vehicle was badly flawed in a number of ways: Loading and reloading the gun was difficult to accomplish. The crew layout was really bad. The commander operated almost blind unless he remanied out of the hatch. The use of the remote machinegun on top was hard to use, harder to reload, and if in the wrong position, a clamp on the loader's hatch trapping the crew in an emergancy.
Visibility in general was poor. The gun had a very limited arc of fire. In fact, the worst of any German standard SP gun. All-in-all it was a poor machine.

The Elefant and JadgTiger: While the former did have an impact on the war neither was built in sufficent numbers to really merit inclusion on a best list. Both also had serious problems mechanically that would preclude them on technical merits; the Jadgtiger moreso than the Elephant which proved the better of the two vehicles by a large margin.

Another possibility is the Nashorn. While it has paper thin armor it does have an 88/71 at a time when that gun could reach out beyond anything the Allies had in the field. It saw sufficent service in sufficent numbers to warrant at least a mention and possibly an inclusion.

The Jadgpanther should make the list on its technical merits only. Its performance in the field was brief and proved less than stellar. It had the potential but it never really got to shine showing up so late in the war.

The SU 85 and 100 are likewise good designs. These too likely warrant a spot on this list for their clean lines and good design. The 100/60 is also a very potent weapon.

The Jgpz IV is an overloaded vehicle with marginal performance. It like the Hetzer is built cheap and lacks the necessary refinements to make it really effective in the field. There are at least a few of these vehicles where the crew improvised a cupola by welding on one off of Pz IV or Panther onto the vehicle.