Considering that a majority of legends are duel types(I'm excluding Arceus seeing as it can shift between types), the primary type mainly being dragon, it can be hard to say. But I would probably say it's bug, Genesect is the only Pokemon that comes to mind when I think of bug legendaries, it's type being bug and steel. Secondly, the only ghost type legendary I can think of is Giratina, making ghost another type not commonly used. Of course there is the new types that are to come in the next generation, though I'm not quite sure.
I hope this answers your question :)