Not sure I can name the absolute worst I’ve seen, but I can name some ones I either despise or don’t care for. Most bad ships I’ve seen involve incest (Elsanna, Loudcest), but as for ships involving two different gene pools…
Tigra / Cheetara (ThunderCats 2011): She ended up falling for that jerk just because he was nice to her one time when they were kids. Wow, what a bond! No way that was done just to create drama with a love triangle!
Ned Flanders / Edna Krabappel (The Simpsons): If it were early season Edna, I would’ve been for it, but it was slutty, bitchy, Al Jean era Edna, so I hated it. I couldn’t stand seeing her happy. (…is that bad?)
Miss Martian / Lagoon Boy (Young Justice): He was nothing more than a rebound guy and we all know it.
Mai / Kei Lo (ATLA graphic novels): Same as the previous one. Glad he left in the end. (MAIKO 5EVER)
Duncan / Gwen (Total Drama series): In fact, I don’t like seeing that douche with anyone. LET HIM SUFFER FROM LONELINESS FOREVER.
There may be some others I’m forgetting, but these are definitely ones I don’t want to hear spoken of around me in a positive light.