With "milyen~what kind" and "melyik~which", do we have : Milyenek epuletek magasak ? = what kinds of buildings are tall ? (e.g. hospitals, banks, as general statements) Meliyk epulet magas ? = which building is tall ? (e.g. the one on the right, specific statement) Making the proposed translation "What buildings are tall?" a bit confusing. Unless I am mistaken and I don't get the meanings properly :-?
Also, your "Milyenek epuletek magasak?" should be Milyen epuletek magasak?. The word milyen behaves like an adjective - when it comes before a noun, it doesn't take any endings. But if it's playing a predicate nominative role (like Milyenek azok az épületek?) or otherwise acting as a noun (like Milyeneket találtál, "what kinds (of things) did you find?") then it does.