Why not Milyenek here?
Milyen behaves just like an adjective, which makes sense because it basically substitutes one. So if it's attributive, standing directly before a noun, it's always singular, you don't attach any suffixes.
"Milyen buszok a nagyok?"
Névelővel nem jó?
This sentence is structurally the same as "What buildings are tall". See the fairly long comment on the many possible colloquial variants of this construction!