Axel definitely, Axel.
To calrify it for everyone the reason Axel sacraficed himself was A. to show off to Sora cuz he likes to brag, and B he and Sora were getting tired of all the nobodies.Plus, I,ll tell you this Demyx is cowardly,and is a wuss most of the time and always get's freaked out whenever your about to fight.
For instance.
Sora,Donald,and Goofy see Demyx
Demyx: Well whaddya know It's you!
Sora: Hey, I remember you!Your the theif we saw in the underworld!
Demyx: Hey,easy with the words!
Sora
fft what a wuss how'd a kook like you get into OrganizationXIII?
Demyx: O_O (Is hurt by Sora's words) I told them they got the wrong guy!
Sora: Get out the way or else!
Demyx: (Squeals like a little girl) Umm... SILENCE TRAITOR!
See what I mean Demyx has no backbone while Axel is pretty much hard to the core.One look at Axel and I bet he'd freak out.Plus,Axel's flames are so hot that he was able to keep them up while fighting Roxas the entire time,plus looking at the temperature of the flames they're probaly the same at least 97 o degrees,and you'd need at least 10 or more water hoses to put out his fire.Also Demyx's attacks are not that hard to take down,All axel has to do is grab one of water clones and swing them around wildlythe other clones like how Sora did.End of story.
Not that I'm being a fanboy,Demyx is the weakest of the Organization.