Forum Discussion
For me it is:
Eco Lifestyle
The NAPs ruined it for me and that it's such a hassle to turn them completely off when you play an older save.
Worst are the ones where NPCs steal the stuff from my lots and the one where unplayed Sims 'get touchy'. That's just an absolute no-go without consent.
But polluted Evergreen Harbor is still quite cool :]

Get Together
This pack is just not my thing and I should have looked closer before buying.
Usually I don't think twice buying a pack with a new world, but San Sequoia was utterly disappointing for me.
I was so excited for a new city but it just has nothing that makes us want to live there.
Two of the three neigborhoods even look the same -.-

And Seasons
I do enjoy the snow once a year but this pack totally ruined my gardening fun.
Before Seasons each of my households had a garden and I loved gardening so much.
But now with having plants that just don't grow in certain seasons and ruin the 'design' of our gardens...
I'd have to switch them or build greenhouses on lots (that are too small anyway), ouf...
If I build gardens/greenhouses now, I put more 'fake' plants between the real plants ^^'
So if some of them are out of season, the garden is still pretty.

A pack I regret buying would be tiny living. Building a home within the requirements is more annoying than I thought it would be and I pretty much never use any of its items either.
A pack I want to buy but i’m scared I would regret would be seasons. I’m really worried I would get sick of the seasons changing after a while and unlike storms and other weather effects or removing holidays there’s no way to turn season changing off other than using the weather machine to skip certain seasons. (I’m on console and can’t use mods before anyone mentions those) but other than that I love the cas items, builds items, the new careers and the new plants which makes me want to buy it :/
Edit: So apparently the seasons could be toggled on and off in the sims 3 but not the sims 4..why?