I don't have pets yet but I'm pretty sure it will be my favorite! Ever since the sims 3 came out I've been waiting for this expansion release after release.
But, if I were to pick from the ones I do have, I would probably pick WA.
It just adds alot more to the game. (Worlds, vacation homes, skills, tomb raiding, objects, outfits, sims, opportunities, etc)
If I had to pick a least favorite, it would be generations. I simply don't like playing with large families an dthegame usually ends for me when the 2nd or third generation is born so I don't really get to enjoy many of the features.