Favored enemies don't make a whole lot of sense. The way this game is designed, with the constant increase in a player's power in comparison to the game world, the favored enemy is useless.
The ranger got bonuses against giant class monsters in AD&D. These monsters were always far stronger than the average low level character so they were seldom used early on. As players got stronger they were able to compete with these things more readily and they had an advantage over other characters of the same level.
3e made it a class feature that the ranger could choose from a list of potential threats to gain this advantage over, but they dropped the ball in several ways.
First, the player got significantly higher bonuses against this enemy as they leveled, while at the same time, the campaign moved away from these monsters as viable foes due to their CR.
Second they forced the DM of the player using the ranger to include these foes in the setting or the poor fool felt cheated.
Third the ranger is a woodland based tracker and hunter not some goblin sniping, orc mauling menace to all that is deemed unworthy. These things might inhabit sections of a game world but they certainly aren't the only thing there is and when it's time to gain a new favored enemy they aren't going to disappear to be replaced by the player's newest flavor of hated enemy.
Instead of favored this and favored that, in the character description, this kind of single minded devotion to the complete eradication of a species might be better served as a theme where anyone who feels the need to hunt down and destroy a creature type be given the feats to do so. It shouldn't be the ranger's job unless she wants it to be.
Rangers should be good at wilderness survival, hunting, and tracking. They should be so in tune with nature that they get one or two druid spells a couple of times a day once they reach a significant level, around 10th or so, and never get spells that are designed to replace the character's weapons, enhance them maybe, but not cause damage themselves.