Horrendos: Let's start with why you want a PrC, and then move to what kind.
Reasons for taking a PrC:
1) To add a capability to your PC that he didn't have before. For example, Shadowdancer gives you interesting stealth-related abilities (Hide in Plain Sight, Shadow Jump) that the ranger normally doesn't get at all or can't have until very, very high levels.
2) To specialize your PC in a particular area. This doesn't apply so much to the ranger, as it's already a relatively specialized base class. However, there are some ways to focus the class a bit: Being a better archer may be the route you'd like to take, so Order of the Bow Initiate (Complete Warrior), Peerless Archer (Silver Marches), or Deepwood Sniper (Masters of the Wild, or MotW) are options for that. Perhaps you want to focus on your favored enemies; Foe Hunter (MotW) can help with that.
3) To add game-mechanical "crunch" to a role-playing decision. For example, the Witches of Rashemen, the Knights of the Middle Circle, and the Purple Dragons of Cormyr are all organizations that a character can join. PrC levels in the Hathran, Knight of the Middle Circle, and Purple Dragon PrCs represent membership in those orders. Likewise, a character who chooses to become a Stalker of Kharash or Master of the Yuirwood dedicates himself to the wolf guardinal lord or the mighty forest of Aglarond, respectively.
So, in sum, it depends on what you want. More info about your PC would be a REAL help.