Pfffuit, a lot...but I guess the worst I know is the typical first chapter with a rape, and two chapters later, the characters love each other. The worst because it's horrible, illogical, and generally the story has a bad plot ( yeah, there is Viewfinder, but the relationship isn't concluded so easily so it's not really the same)
Oh, sorry, I thought you talked about clichés only, not a specific manga >.<"
I've read a lot, but it's hard to remember all of them. Despite being cliché, they're cliché to the extent of being funny so I find them still enjoyable with the right mindset, like:
http://www.mangago.me/read-manga/fukurou_kun_to_kare/
http://www.mangago.me/read-manga/kanegura/
http://www.mangago.me/read-manga/bull_to_arukeba/
Well, Junjou Romantica of course. But i love it, because the manga knows it's cliché and make fun of it, so it's really nice. But I you want yaoi clichés, I guess Junjou is the best... ╮( ̄▽ ̄)╭
What is the most cliché yaoi/shounen-ai manga you've ever read?