Correct translation would be : Dans tes dents, John Doe

“Suck it” with a vengeful connotation could be translated by “Dans les dents”, “Dans ta face” (both mild), “Dans le fion” or “Dans le cul” (both very vulgar).

“loser” could be “minable”, “minus”, “nullos”, “bon à rien”, “gros blaireau” and many other variants depending on context and the age of the person saying it.

Without knowing the context, I can suggest “Dans ta face, minus !” ou “Dans le cul, gros blaireau !”