User blog comment:B214/Question and Answers 2/@comment-86.81.216.93-20140531103621/@comment-73.178.133.185-20140601035852

Depends on what you mean by "strongest"? Attack power? Skill? Endurance? Don't give me general, because its too broad. The results of a fight between Xenovia and Kiba will vary greatly depending on the circumstances (place, time, reason for the fight, emotional state, etc.) It's the same for the ranking the members of the Occult Research Club from strongest to weakest, which is being discussed above, with the added issue of compatibility -- someone who can defeat Issei may not be able to defeat somebody Issei can defeat.

Although Kiba has the power of Gram now, Xenovia still possesses the greatest destructive power. Currently, Kiba will still come out on top 8-9 times out of 10 in a fight between the two of them, but with Xenovia learning how to make full use of ExDurandal (particularly if she masters Ex-Ruler) her potential has gone up quite a bit. But at the same time, Kiba isn't just sitting around satisfied at his current level and is trying something ridiculous to further level up. Yet, in the end, Kiba's flaw is still a huge one -- any successful attack by Xenovia would easily overwhelm Kiba's defenses and bring about his defeat.