Omegahugger
Inattentive Idiot
I would assume that whoever Mathilde ends up dating would have the most narrative relevance on account of dating Mathilde.I've said it before and I'll say it again: one of the most important things to consider for a quest romance candidate is how narratively important that character is, i.e. how likely it is that we'll want to do things together with them. A character with high narrative importance will naturally appear a lot and hang around Mathilde (or Mathilde will hang around them, same difference), making for easy scenes that have romantic subtext but aren't just about romance. A great example is Natasha in DADA, as she has great involvement in almost all that Frederick does. If a character does not have narrative importance, they are likely to be relegated to occasional romantic scenes that are otherwise completely divorced from the rest of events.
Out of the current top 3, it is my opinion that Johann has the greatest obvious narrative importance as both adventure buddy and fellow researcher. Cython's is also pretty good, because even if we don't romance them, we're still likely to hang around, and they are generally fascinating. Cython is probably not super likely to involve themself directly in stuff Mathilde does... though that might change depending on how strongly they want to involve in Mathilde's life. Panoramia is in my mind the weakest narratively - she spends all her time doing agriculture, which Mathilde has no reason to participate in, and doesn't seem inclined to venture out or engage in research. The only real meeting points are, potentially, Waystones (and then she's not likely to be the prime specialist we'd contact, being a Journeywoman), and defense of K8P if something attacks it.
Out of the other candidates, Roswita seems to have pretty good narrative relevance, since we do care for Stirland, Anton has less, and Oswald has barely any.
Like, the main love interest is not a role that usually fades into the background, and when they do it's usually a sign of poor writing.