Never bothered watching Love Alarm and have no interest in it. But yeah, Boys Over Flowers was the first Korean drama I ever watched and I despised it. It accentuated all the issues with the original story, making what was already often shallow, annoying or problematic all the more so.
Personally, the worst one I watched was the Liar Game Kdrama. I've heard the manga itself never got close to reaching what it aimed to do so that likely does a play a role here. All I remember from the show is that the characters were unlikeable while making dumb decisions, the plot didn't make sense and that the ending felt anti-climactic.