Has anyone found an allergy medicine that's very effective at treating allergies to cats?

Cat allergy. Most patients allergic to cats will do well with zyrtec, xyzal (levocetirizine) or allegra. However, depending on how sensitive you are, you may require the addition of nasal steroids or antihistamines and/or singulair.
Difficult. Cat allergy can be very difficult. In general the treatment approach is avoidance, medicines and immunotherapy.