There are just manga you avoid. You hate the art, the story line, the characters but...you pick up a manga that looks...different. You didn't plan on enjoying it but now it's your favorite!
Surgeon's Law states that 90% of things are crap but that just makes the 10% worth dying for. What is the mangas that are the exception to your rules? Not guilty pleasure but a manga that you tell everyone about even if you rather despise a certain genre?
For me it's Usotsuki Lily. I am usually not a fan of shojou but this is funny, the cast is great and there is a ton of lampshade hanging on shojo tropes. Maybe it's because it's more of a comedy manga [like Ouran] or maybe it's the crossdressing.
Note: I was going to post this in shoujo but I want to hear about all demographics.
Noblesse I picked it up as it was the highest rated series on MU (around a year ago). Then I dropped it like 3-4 times before reaching first 50 chapters seeing the artwork and slow story progress in the start.......And well now it's my favourite series!!
Artwork greatly evolves later on and now it has one of the best artwork. The story might not be the most complex one but the characters are. They are both original & interesting.... A must read!
One Piece Didn't liked much at the start and dropped it 1-2 times possibly because of the artwork. Now one of my favourite series...
Oh, Boku wa Tomodachi ga Sukunai is the first thing that comes to mind. I have a strict rule against harems that are harems for no clear reason other than to be a harem, and a rule that I'll refuse to read anything that has a male protagonist who establishes a harem without having a single, relevant male friend, but I let it slide for Haganai cause it's at least a relevant plot point.
It's admittedly getting harder to swallow the more I read, though...