"What do you eat for breakfast?"
Translation:Kion vi manĝas por la matenmanĝo?
According to PIV, it is -- matenmanĝi is intransitive and so it can't take an object.
It would be like asking in English "What do you eat breakfast?" or "I eat breakfast bread" -- "eat breakfast" can't take an object either.
I suppose you could try "matenmanĝi je io" ("Je kio vi matenmanĝas?").... but I wouldn't recommend it :)
What is the "best" vortaro? According to reta vortaro, matenmanĝi is transitive and could take an object (in German, e.g., it's also transitive. Basically, "to breakfast something" is a verb.)
For what it's worth, Duden's example of "ein Brötchen frühstücken" for the transitive sense of "frühstücken" sounds extremely odd to me.
As to what the best vortaro is - good question. There is no "official" dictionary which defines what is and what is not correct.
I tend to go by PIV, but it's not the only one and not definitive.
I would tend to distrust Wiktionary. But as for ReVo? No idea.
As a native German speaker "etwas frühstücken" ("to breakfast something") is very normal and natural sounding. But equally natural is "etwas zum Frühstück essen" (to eat sth. for breakfast). On the other hand, "etwas abendessen" ("to dinner sth.") does not exist. So it's not consistent - but Esperanto's goal is to be consistent.
So I don't see why manĝi is transitive, but matenmanĝi should not be transitive.
It definitely is more consistent than other languages - and for your examples, of course I have no idea. Even though I found that you could also say antaŭ kiam and post ol, just that it is unusual.
Esperanto's goal is to be consistent? I'm not so sure of that. It's not perfect at it (why is a brush a "broso" but a comb a "kombilo" if the verbs are "brosi" and "kombi", for example? why do we say "antaŭ ol mi faris tion" but "post kiam mi faris tion"?).
“… por matenmanĝo” would translate as “for a breakfast.” If you speak about the next breakfast or the breakfast in general it is better to use the article la. In English it is often left out because you have the possibility to use a.
But in the question it sounds like "what are you regularly eating for breakfast". I would be fine with saying "la matenmanĝo" all the time, knowing that it's the only way to phrase it, but using it without the definite article makes sense to m.