I'd say 85% of the anime i've watched is just meh for me, even the "good" anime I just can't get into. Maybe its a cultural thing, how japanese structure their stories compared to the west or how the industry is like a sweatshop and not much room for creativity is allowed? No idea. Worst anime i've seen? I think a few, though I dunno if its bad so much as numbingly boring.
Monogatari I tried watching and I gave up maybe 4-5 episodes in because it was just philosophy spewing and tiddie monologues. Whatever plot there was was overshadowed by dumb anime exposition and """aesthetic"""".
Chuunibyou demo Koi ga Shitai! was a bore fest. I thought it was about fantasy stuff but its just mind draining slice of life. Bleh.
And the worst out of all of them i've seen I don't even know the name to, I just watched it because it had a seal mascot. I think it was some afterschool moe anime and i'll credit where credit is due, the two others had a theme to them, this... this was like staring into a black hole. It felt barely like it was anything, it was just going through the motions, it was so bland it made white bread look flavorful.