You can pick up well over 1,000 reputation by wearing each faction's tabard into a level 85 dungeon; heroic dungeons grant a little more, of course. Some factions have daily quests that will help you fly through the ranks even faster. But the tough part can be deciding which reputations to work on first.
Let's take a tour of those factions and see what kind of epics or notable gear is offered by each. If you see something that's especially appropriate to your class or role, that's probably a good hint that you should start there.