One of my worst experiences would be with an R18+ otome game called "Toiro Komachi ~Kurenai ni Somaru no Toki Made" (http://vndb.org/v3426). It's not like the worst game ever made, but it's one of the worst otome games I've ever played. It wasn't even "so bad, it's hilarious" or "so bad, it's good." It was just BAD. Extremely long common route and way-too-brief interactions with the love interest of your choice during the specific routes. It feels rushed, with a few unresolved plot points and characters changing tune at a moment's notice.
Out of curiosity, I played an eroge which is currently the lowest rated VN on VNDb.org, called "Shiyouchuu ~W.C.~ (a.k.a. Water Closet: The Forbidden Chamber)" (http://vndb.org/v1484). Unlike the above game I mentioned, though, this one had its funny moments and wasn't long enough to really get sick of it. Strange interface, boring music, subpar voice acting, dodgy art, sporadic pacing and practically no story... In my opinion, "so bad, it's hilarious." It deals with scat fetishes, so if you're iffy with that stuff, I don't recommend checking it out.
Other noteworthy bad experiences include:
~ Love & Order (the worst otome game I've ever played, period): http://vndb.org/v6267
~ Yandere (in fairness, some parts were quite amusing but it's a bad game overall): http://vndb.org/v1303
~ Maiden Rape Assault: Violent Semen Inferno (too bad its title, "violent semen inferno", is pretty much the only entertaining aspect): http://vndb.org/v1292