I start with my 'vote'. Hortobágy National Park in the NE part of Hungary is considered the best place to find or photograph birds. The number of bird species is improbably beatable in any other part of the country.
Many attractions are the the lovely Squacco Herons in breeding plumages during spring. Also Glossy Ibises, Red-footed Falcons, European Rollers could be find within the boundary of the park. During fall large numbers of Common Cranes cumulating on the feeding areas as well as the night roosting habitats. Lesser White-fronted Geese visit the area in decreasing numbers but Greater White-fronteds are fairly common.
Entering protected areas requires permit. Aslo you need to mention you plan to photograph birds.
Nice option is to try one of the bird photography hides. I am not including details here as it would be considered as an add but upon request I can provide contacts.
Find the location on Google Maps!
Szimi