I am a curmudgeonly old fart. There are a good 100 manga I have disliked enough to stop reading. I'm just glad that I get review copies from emanga, because that has saved me a billion dollars (not really). Now if I really like something I go buy it. It used to be that I would flip through copies at the store until I found something I liked. Also...libraries have more manga than they used to. I like that.
My name is Dr. M, and I have read some bad, bad manga. It's okay--only a few have scarred me for life. Sometimes that which has been seen/read cannot be unseen/unread.
Here are some things off the top of my head:
Mr. Tiger and Mr. Wolf- it's like the set up the premise only to forget it two seconds later (also, might be some sort of furry-hentai, but I can't tell...my inability to tell is a problem too. It could go either way.).
Prince of Ramen-Actually worth a read, if you can handle page after page of really obvious puns and over the top ridiculousness. Provides some much needed "Perfect Cup of Noodle" advice (pictured below)
There are other things, certainly, but I try to forget the awful stuff. I don't ever read it through entirely when it's bad.