Out of the choices given, MobyGames. In terms of release information, credits, cover scans and tech specs, even today I don't know of any other site that even plays in the same league there. But I have a lot of databases and resource sites bookmarked which I, when researching a game, usually "Open all in tabs".
Most of the really popular sites are crappy. IGN, Giant Bomb, even AMG are usually very incomplete, have a lot of flat-out wrong information, and are very limited in their scope. Terribly unreliable overall.
There are some great sources which are undeservedly not better known. Some of my favourites are:
Have a look at them!
