Lana

Lex needs to stay away from romantic relationships with women. The relationships tend to be toxic, and in Lex/Lana's case, they end up abusive. I may love Lex as a complex antagonist, but the way he behaved during his relationship with Lana was so revolting. People harp on about how vengeful and morally dubious Lana became in the second half of season 6 and in season 7, but if you look at what she went through at the hands of Lex, it makes sense that she became hell bent on seeking revenge.

And anyways, Lex's relationship with Clark was easily his deepest, more complex relationship, besides his relationship with Lionel. And honestly, Lex and Clark's dynamic felt far more romantic than any of Lex's romantic relationships with women.