As with Roose Bolton in the previous poll, I find it weird that Arianne Martell gets any votes. Her ability only triggers when you lose the battle. Losing battles is generally not a good thing.
Which is why Arianne Martell is such a good card ... you WILL lose battles so blocking another house from taking ownership of a vital territory is huge (especially if you're higher on the Iron Throne track and can regain it before they can move in to it).
I prefer other Strength 1 House Cards but this one is a very good card.