I mean, worst can be used in many different contexts here. Do you mean worst as in quality, playability, production, etc? I could think of many sets that fit one or more of these criteria. Might as well go on a ramble about a few sets that got me.
XY-Flashfire was a set I'm personally not fond of. Set had pretty bad art, only a small few playable cards, and was overall really made to pander to Genwunners with, what was it, seven Charizards? The set felt unnecessary if you want me to be brutally honest. It was sort of a filler to Furious Fists, which was, although one of the lesser sets, still felt like it was more of a main set and not complete filler.
XY-Evolutions was another set that felt kind of lazy. The set added next to nothing to the game, and of course, more Gen 1 fanservice.(I do realize that it was for the 20th anniversary.) It was a nice touch to see a rebirth of all the original cards, and a new look on some older cards(Giving the Kanto megas Spirit links and full arts made me somewhat excited at release, as I always wanted to try out Mega Venusaur's attack as a fun deck). The issue I have with it is that, being the last set of XY, you'd expect something more like Black & White-Legendary Treasures to be the sort of "celebration" of what the era has given.(My only slight issue with LTR was that the EX's were straight reprints, though it's not a huge problem, I liked a lot of the BW-era EX card artworks, especially the ones by Shizurow, so it wasn't a huge deal). Maybe it was for the better though, that we didn't get certain cards in for longer. It's quite conflicting when you think about it. The XY era feels like it was left somewhat unfinished, but at the same time the metagame can't stay stale, we need to move onto newer concepts(And trust me, I'm quite excited to see what the new GX era has in store). I guess, if anything Evolutions should have added more to the game instead of being pure fanservice. It's really not even fun to collect, as nearly everything in it has been done before. If anything that was the set's biggest issue. Although Generations did straight reprints, all the Ultra Rares were new, there were some viable cards, and the pull rates were great. If anything, they could have made Evolutions and Generations one and the same, and it would have been a way nicer set, either as a main or a side set.
I don't know if this is my own issue, but most of the cards made in XY-Fates Collide were a lot lower quality than cards made in the past. The foiling effects were see-through, the cards in general felt....Rougher? It's hard to explain. Maybe the printing company they went to was just worse or something. Not a problem with the set, the set itself was alright but it was definitely the most poorly-made of the era in terms of card quality(but I bet I'm really the only one who cares about this.)