What's your least favorite expansion pack?

Both. Uni is destroyed by endless boring homework. IL has no gameplay at all.

Uni is horrible. Did it once, and was bored out of my mind. At least with high school, we got boba tea and creating outfits.

Eco Lifestyle gives me mixed feelings. The Evergreen Harbor World, blah. :s The furniture, windows, doors, pictures & plaques, great. :p
Now as for Snowy Escape I really like everything in it. B) The contrast of Winter & Spring with the beautiful Asian decor of the Mt. Komorebi is wonderful. Plus I so love the winter snowy Mountain. <3

I love all the EPs I own. Yes, there are some things I could criticize here and there, but the parts I enjoy about them are features I get a lot of mileage out of in my game play.
For my most disliked pack I chose a pack I don't own. Had a hard time deciding between Eco Lifestyle and HSY. I decided I wasn't going to choose one of the packs where I'm not interested in the overall topic. Never owned a cat or a dog IRL, never understood the appeal. I can't say I dislike the Cats & Dogs EP, though. I'm just "Meh, not my thing."
With HSY, on the other hand, I was really disappointed. For me, the part of my own high school years that I liked the most was the time after school, hanging out with friends. Before HSY I kind of had this idea of a Teen (not high school) Game Pack that would have teen social groups functioning similarly to clubs in the Sims 2 Free Time. HSY gives us a tea shop, which is not really ground breaking since we already have coffee shops. You can be a streamer, but didn't we already get that in different forms in both Get Famous and City Living? The after school clubs are mostly rabbit holes. It's a disappointment.

I'm not sure if this counts or not as I have never played it, but Eco Lifestyle is the only EP so far I have no interest in owning, and is the only one I don't own, so I will say that, because I love the others. :)

Of the EPs I own (all except HSY, Growing and HR), I like DU the least. As others have said, the system is too much of a grind. I find living on campus boring and also disruptive to my style of play, which is usually legacy. The world is also boring and is the one my sims visit the least. Plus, I hardly use most of the bb, which is kind of ugly. I get the most use out of the cas; at least I can dress my teens in it and they look less like adults.

I really love ecology and sustainability but the way it was developed in that EP didn't really attracted me, and Snowy Escape it's really beautiful and well made but i feel like the gameplay it's not for me.

It's a tie between Get to Work and High School Years.

With Get to Work, its CAS aged horribly, the retail system doesn't work, and two of the three careers are bugged. It's such a downgrade to Ambitions and Open for Business. I tend to give it a pass compared to newer packs, but even the EPs I dislike have some stuff I use.

With High School Years, I just like the world. Sometimes. Don't really play in it all that much since it's so small and the lots are puny, but the wharf is fun. But Social Bunny is annoying, the CAS is fine but a little bland for my liking, class is boring and adds almost nothing, and there's not much to do. The Entrepreneur skill doesn't stand out as anything but a way to make money, in a game where it's already too easy to make money. The sneak out feature was so poorly done and hard to work with. But while Trendi isn't something I use often, it's fun to play with when I do. I also like the art deco items and use them a lot. I like the aspirations and pranks, wish we got more of them

For me it's Horse Ranch horses pet objects and wine making does not make an expansion.

High School Years.

I do like the updated animations and the new voice lines for teens and the cas and bb but there is nothing to do at the high school, you get a few tasks like talk to a student and a teacher. Classes consist of watching your sims looking at the teacher talking. No active classes, no goals, you don't need any skills, only a random chance card shows up sometimes and after school activities are just rabbit holes as well as the amusement park which is not an actual lot. They didn't implement anything like what we saw in Get To Work or Discover University where we had things we could craft, unlock, different tasks, active organizations. We can't choose which sims that show up to the high school either.

Everything seems to instead focus around prom with it happening every week, but the actual prom event is too short and the "showing up before prom" and "after party" event consists of your sims just standing still doing nothing. The new social bunny is good for sims to keep being friends without seeing each other but the app also spams random messages.

Apart from that, we get a few new rabbit hole careers and we can create outfits and drink tea at a thrift store which is ...fine but not what I expected for a pack that was supposed to focus on the high school part of our sims lives.

But I also would like to say that I think a more proper word to use than "dislike" is "disappointed" in my case, I was really looking forward to this pack and was very disappointed with it.