We have visited all the countries that you mentioned and loved them all.
First, I lived in Augsburg, Germany for four years and did the Oktoberfest three times. Book your lodging now. If you can't find lodging in Munich, book it in Augsburg and take the train down every day. Don't miss the opening parade with all the beer wagons. Get there early to be able to see it well.
As for choices for which country, you need at least a week to do Poland properly, visiting Gdansk, Warsaw and Krakow as a minimum. You need 3-4 days minimum in Prague and several days in Budapest and Hungary.
Greece to see Athens, Delphi, Corinth and the surrounding area with some islands close to the port you need a week there. If you visit islands, go to Santorini, Mykonos, Rhodes and Corfu (if possible Crete). You could spend a week alone in Crete, but you need several days to do the other islands.
Turkey, suggest doing Istanbul. We did it well in five days. Do visit Hagia Sophia the 6th Century Basilica, a portion of the ancient walls were repaired as well. DO the Blue Mosque and the Hippodrome. Recommend a hotel close to the Hippodrome. Visit the famous Bazar and Topkapi Palace (Sultan's residence). The Archaeological museum is fantastic. There is more to see but you need five days.
If you stay in Germany, Poland, Czech Rep. and Hungary, you can rent a car to see the countryside like visiting Dresden or take the trains. You don't want a rental car in the city. Look for local day tours to go outside the cities.