In D&D 3.5, the ranger favored enemy bonus is no longer a precision-based bonus, so it will work against creatures that normally cannot be affected by a critical hit (undead, constructs, plant creatures, elementals, etc..)
This means that picking undead as your favored enemy is definitely the way to go. There are more undead type creatures in D&D than any other subtype.
In D&D 3.5 you can also pick humans even if you are not evil, so alot of rangers will choose this option. My DM hardly ever uses humans though, so I didnt pick it.
Giants are always a popular choice, especially if you plan on being level 9 to 16 for quite a while.
In D&D 3.5, outsider-evil is now an option. This covers all kinds of creatures, including devils, demons, yugoloths, and other assorted fiends.
If I was creating a 20th level ranger for D&D 3.5, here are the favored enemies I would want the most:
1. Undead
2. Magical beasts
3. Aberrations
4. Evil-outsiders
5. Giants
About 70% of all creatures fall into the above five categories.