I'm pretty happy to treat the official subtitles on a DVD as being about as good a source of information as you're likely to get. They're (usually) added by professional subtitling companies and organisations, often with direct access to both screenplay and the production team who can answer any of their queries about spellings and translations. Although errors can potentially creep into any secondary source (especially where subtitles were added decades after the film was made, for example), the reality is that these mistakes are unlikely to be egregious.
Most professionally constructed English DVD subtitles are, within the bounds of readability, likely to be a highly faithful reproduction of the work.
Foreign Language Subtitles
With any translation, there are going to be choices made by the individual translator. A faithful translation would be almost unwatchable so local idioms need to be used to replace those that wouldn't make any sense. This in turn means that the translation isn't going to be a completely faithful rendition of the original but is still likely to be of a very high quality.
Caveat emptor if you plan to use a re-translated source in your answer.
Fan subtitles can (like any of other source of fan-created info) be used as part of an answer but always with the proviso that they're unlikely to be of the same quality as official subtitles and that errors of assumptions or attribution (for example, who spoke a line heard off-screen or the precise spelling of an alien word) may be made.
Fan-subs are low quality sources and should be treated with caution.