You're joking, right?
Did possibly the Germans not notice Poland's signature on the Geneva Conventions?
Or Czechoslovakia's? How about Latvia's, Estonia's, Serbia, Croatia, Slovenia, Greece or Hungary's signatures?
And let's see, would Malmedy in December 1944 be a good example of German "correct behavior" towards POWs?
Yes, somewhere I did read that about 95% of western allied POWs survived in German POW camps -- but that did not include Jews or blacks.
So I conclude that "correctness" in the German military mind was a matter of highly subjective interpretation.
Malmedy wasn't Wehrmacht. It was SS (note my careful use of the phrases 'German military). And that puts aside that pesky little piece of evidence that some of the prisoners made a break for it causing the guards to open fire.
As for Poles - yes, the Germans were much more correct in their treatment of Polish prisoners of war than were our Soviet allies. But let's look at that in a bit more detail. Recall the American treatment of German POWs in World War II. Until the eradication of the German government, the captured soldiers were treated as POWs, with all of the rights that come with that. Immediately after the surrender, they were reclassified, and stripped of many of the rights that they had enjoyed.
Similarly, the Germans initially treated the Polish POWs with all of the rights of that status. (In fact, many from the eastern portions of that country were repatriated to the Soviet zone, some from the western zone were released). In February, 1940, after Poland ceased to exist, they converted the enlisted men to civilian status, and offered them the choice of becoming civilian workers or internment in civilian camps (with the Jews, of course, passing to SS control). (Officers were still treated as POWs. Indeed, the bulk of the Jewish officers survived the war, although they did suffer rougher treatment after their separation from the Catholic Polish officers in 1940) Some numbers and estimates here: http://www.zchor.org/meirtchak/biblio.htm
Latvian and Estonian? are you referring to the traitors who joined up with the Red Army after the Soviets overran those countries? To the extent that they served in the Soviet army, they don't qualify as being 'from countries that were parties to the Geneva Convention'
Serbia, Croatia, Slovenia? They weren't even countries at the beginning of World War II, and weren't in 1929 when the Geneva Convention came into being So they, by definition, couldn't be parties.
Hungary? They were a German ally. At least get your sides right.
That only leaves Greece from your list. If you have any documentation of widespread mistreatment of Greek POWs by military units, I'll be happy to look at it.