I would say look at my visual novel list and see what I rated three and under, but I realize that some of my low votes are due to my preferences. So here you have it, the list of VNs that I would tell most players to stay away from and why:
1. DOR - A collection of three weird and pointless stories written for the sole purpose of the sex scenes you get at the end.
2. Dosukebe Futanarikko ga Hitome wo Nusunde Oogoe de Senzuri ~ Asa kara Zusshiri ☆ Milk Pot 1.5L - Three things happen in this VN: 1. A rich futanari girl jerks off. 2. She becomes an Internet idol for some random reason. 3. The screen turns white and she asks the player (or somebody, it isn't really clear who she is talking to) if they will help her accept herself. It is very weird, and was know as the worst nukige ever written until Dual Phase Soukakurou (Anal mo Vagina mo Hirogacchaimashita) came along.
3. Yu. u. e. n. chi - At first, this just seems like a generic romance/slice of life VN. That is, until you discover that the central conflict in every route, I repeat, every route, is the same.
4. Netorare Oku-sama wa Shojo Kousei ~Ryakudatsukan~ - A teacher finds himself attracted to one of his female students. After he discovers that she is married to another teacher that he hates, he blackmails her into becoming his sex slave. Boring and has some extreme fetish elements if I remember correctly.
5. Dual Phase Soukakurou (Anal mo Vagina mo Hirogacchaimashita) AKA "The Worst Nukige Ever" - The story of the sex-filled life of two loli twins. To complete it, you have to hear the story three times in a row (from different perspectives), which is painful considering how bad it is. There is a lot of extreme fetish stuff and the story is weird. According to saberger of VNDB, this is "the game that you will snap and break the neck of every kitten when you heard its name."