There are endless sources for all types of media, some can be more useful or informative than others. Whilst this list isn’t exhaustive by any means, it could provide some helpful pointers to where you can find new or old information.
In most cases, official sites are the best possible source and contain the most information. Providing these as often as possible is ideal. Especially since these sites should be linked in the External URL section of the form.
News sites are usually fine, but double-checking the sources they reference can be useful.
Media Arts Database is an amazing resource managed by the Agency for Cultural Affairs’ for all manner of information, from animated series, to manga tankoubon, to print magazines dating over fifty years old. Always worth taking a look to see what’s there. (Examples: Dennou Coil, Weekly Manga TIMES)
MyAnimeList, MangaUpdates, and Kitsu are often sourced - whilst this is OK, these alone aren’t always accurate enough, as they can often contain errors. However, please link the first two if they exist, especially as AniList requires the works' MAL ID for mapping series between the two websites for list imports.
Anime and manga aggregators (streaming/scanlation hosting sites) should almost never be used as a single or primary source. They usually lack or have incorrect data.
Moon Phase for airing times, TV stations, etc.
Syoboi Calendar is also handy for airing times and checking delays.
Yaraon! (NSFW) is a pretty nifty blog for announcements.
Anime@wiki for staff positions and episodes.
Anime Staff Database for animation staff positions and episodes.
FanSubDB is OK for airing times, but get additional confirmation.