[PortalTron]

100% Daichi Kamijo. He was one of the most effective wielders, and was also only one to have an upgrade.

Especially given that Daichi has a lot great thematic parallels with Touma as Calibur. He is the "dark" version of Brave Dragon as Calibur, but also similar to Touma in how he tried to stop the future events from happening (by tapping into the power of the books and accessing the Book of Logos). It didn't really make sense for him to get killed off so indiscriminately and given that he had the ability to revive before, he really should come back as Calibur.

On a side note, this might be just me, but I never really liked that they kept having Sophia become Calibur. Primarily because it creates a plot hole where there was no reason why she wasn't designated as a user in the past to avoid causing, well all the events in the show. It also didn't make too much sense thematically given that her becoming a dark version of Saber/Touma doesn't really work in any context. It was kind of like if Ryuga's deck was just handed to Yui or the ORE Journal boss to become Ryuga, which doesn't really make sense for anyone to use except for Dark Shinji. Similarly, (even though he was not the first user), Daichi felt like most understandable choice for Calibur. But that's just my opinion.