Ferrothorn, but even if you choose either one, you are at risk of encountering a Fire type Pokemon which can take 3 of your Pokemon down. I would suggest getting something else that is very slow like Eviolite Rhydon or Swampert. they are slow and very Defensive. a moveset for both would be:
Rhydon-
Earthquake
Stone Edge
Ice Punch
Fire Punch
Swampert-
Waterfall
Ice Punch
Earthquake
Curse