reading english: silently or aloud?

Which is better ( for pronunciation, for grammar, for fluency, for everything)

It depends on the purpose. I think if you read out of interest (with the side-motive of improving your fluency) it is best to read silently for gist. If you also want to enlarge your vocabulary, I would recommend that you sit with a pencil to underline words that you can't figure out from the context, so you can look them up in a dictionary afterwards (I would definitely recommend that you look them up when you have finished reading, rather than interrupting your reading all the time to search the dictionary). On the other hand, reading aloud is good for improving pronunciation, but in that case, I would recommend finding a "listener" who is good at the language, so the they can correct possible errors, otherwise I would find little use in reading aloud, although I sometimes use it to practise diction in a language I can already speak well, without needing to bother about pronunciation errors. Even in this case, however, I would recommend that you tape it and play it back to a partner who could give good tips on diction (a teacher, for example).

If you want to practice reading aloud, try this: find a contemporary
novel and a book-on-tape (or cd) version of it. Then try reading
aloud from the book while listening to the speaker on tape read the
same passage.

Try to notice if what you say has a different rhythm, stress, etc. from
the reader on tape. This will also show you when to make the 'oral
contractions' that native speakers make (like 'have to,' 'got to,' etc.).

Make sure to choose a novel, not a collection of speeches, because
speeches are read in a very different manner than normal conversation.