I'm going to have to argue for Haxorus
Rampardos does indeed pack more powerful attacks, but it is very slow, and with relatively little bulk and a whopping 5 weaknesses, it will often die before it gets the chance to attack. Trick room or rock polish can help with this, but Rampardos will find it hard to find a chance to set up.
Haxorus on the other hand has decent base 97 speed, which can be further boosted by dragon dance, allowing it to fire off powerful attacks before its opponents. With great moves like outrage, earthquake, aqua tail and superpower it can decimate all foes, even the steel types that resist its dragon moves. It's defensive typing isn't to bad either, with only two weaknesses and four resistances. Here's a set I used ingame
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Ability: mold breaker
Brick Break/Dragon Claw
Hope this helps