Depends.
If the idea of revenging Dragonite at +1 through Multiscale appeals to you, go with Mold Breaker.
If you want a touch more power, go with Rivalry, though if the opponent is of the opposite gender, your damage will drop significantly, so take note of that.
If going with Rivalry, make Haxorus Female, since most mons are that now to troll it.