That is completely up to your preference, hence there's several of them (personally, I don't think limiting yourself to a single provider is good idea, but YMMV); what is more important is your preferred providers order, as most of the metadata will come from primary provider (typically TMDB for movies and TVDB for TV Shows, but again, it's up to user preference, some users prefer TMDB for both, others prefer some other combo/order), with missing/additional data coming from other providers in the order of priority. Again, it is all up to one's own preferences.