Worst? I dunno. K-On!! just wasn't my cup of tea despite having very few annoying elements for this kind of show.
I did also try a few times to get into Joe since I'm a huge fan of the manga, but always ended up dropping it. Simply unappealing. Hasn't aged well, and I'm actually pretty lenient what this concerns. :/
Haruhi because Kyon's hype of the characters wasn't justified, since he still remained the only one with a distinct personality after all published LN books. The supernatural also has done more harm than good writing-wise and plot-wise.
I've only seen Cowboy Bebop, I hated it but I think Koukaku Kidoutai: Stand Alone Complex and Mawaru Penguindrum are gonna be atleast as bad since I hated the Koukaku Kidoutai movie and I hate the other two works of Ikuhara that I've seen.