Hm... Picking Remus would be a cop-out, so... Gonna have to go with Euergetia. She's just the most interesting to me. Not only is her story kind of open-ended and doesn't make clear whether she is or isn't dead, but there's the subtle suggestions of deeper behind-the-scenes stuff going on with her that make me feel like there's more to her than we've been shown.
Like... She may have known about the sky??
So much lore left that we haven't seen. Hopefully there'll be another Fontaine expansion at some point.
Cassiodor is my #2 pick though.