Right now the Battle between Girls have become popular from some Anime like Queen's Blade, Kampfer, Sekeirei, Mai-Hime, Ikkitousen and etc. This kind of Anime usually label eechi genre because most of their battle involve getting their clothes tore, fight in a hot spring while naked, having the panties shown while fighting, and for rare occasion getting sexually harass each other on bath or bed(I'm really starting to like it)
Well from all Anime Girls that do Battling each other, I can think of only one that doesn't involve transformation sequence, well-fitted clothes and other eechi stuff or whatever it is Rozen Maiden. Of course Rozen Maiden more an Moe type than an ecchi type(maybe because their dolls but still counted as Girls that do fighting or Battling each other.)
Anyway Anime that have Hot and Cute Babes fighting or battling each other will surely be popular and sell most on Original DVD and BluRay(because in Original there you can watch the Uncensored part of the Anime and its better than downloading the censored part hehe), but most of this kind of Anime haven't end yet so there will be more clothes ripping battle to continue and more fan service hehe.