as much as i like marluxia and zexion, i'm going to go with axel and roxas.
first of all, roxas' skill level is almost if not just as high as sora's, and we all know sora could take down all of the org members. we know this because roxas pretty much had sora beat in their battle, but sora had a cheap shot on him. roxas is definitely the second strongest in the org (xemnas being 1).
YouTube - roxas vs sora
^^^here's the roxas vs. sora fight.
second of all, axel is not weak and a coward. he was ready to fight marluxia and marluxia used namine and sora to get away from axel. personally, i don't think axel gets enough credit (i'm not an axel fanboy, it's just what i think).
YouTube - Kingdom Hearts: Chain Of Memories - Axel and Marluxia II
^^^here's the link of axel about to fight marluxia.
i have to agree that marluxia is one of the stronger org members, but he obviously didn't want to get in a fight with axel, so he can't be that much stronger than, or if he even is as, axel.
zexion is also a pretty skilled org member, as he uses the illusion element. but like someone else said, he was easily defeated by a riku replica, and this was before riku was using xehanort's heartless to his advantage.
don't forget that axel and roxas also have a special bond with each other. some people may not think this has any relevance at all, but i think that certain connections with the person you are fighting with will give you a boost and we all know that roxas and axel are good friends (not gay), and they would fight for each other.
because of these reasons, i believe that axel and roxas would win. (damn, it sounds like i'm concluding an essay or something).