DOKO NO is one of those Japanese things that doesn't translate directly into English. In English we would say:
Which restaurant is good?
But in Japanese they say:
Where restaurant is good?
And this is done using DOKO NO.
DOKO NO RESUTORAN GA OISHII DESU KA? What restaurant is good?
Interestingly enough you can't say DORE NO RESUTORAN to say "which restaurant". You would have to say DONO RESUTORAN in that case but DOKO NO RESUTORAN is the best choice here since DONO is used when there are a selection of restaurants to choose from.
With DOKO NO there aren't any restaurants being selected from. The answer could be: DOKO MO OISHIKUNAI. (no where is good)