For me my favorite is Into the Future, not sure why as I'm usually not into sci-fi but I've always loved that pack.
Least favorite would probably have to be university. I bought it on the off-chance that it might appeal to me, but as soon as I sent a sim there I didn't like the world at all so I never properly played with it. I do like the roommate feature though so I don't really regret buying it. Second least favorite for me is Generations, just because I don't play families. It's the only sims expansion I haven't bought in 12 years, and I don't have any desire to buy it as I don't think I'd use anything from it. :#
