Tbh I would probably LB Arthur first (or Frida if you have her unleveled), he is great for offence and pretty good on defence to.
LB Frosth is waste IMO, he is very good but he really don’t need that.
But if its Alasie or Vela I would LB Alasie, Vela is good on defence but (based on what I saw on few videos) average on offence when Alasie is good everywhere.
It also should depend on which one you are using the most.
Before LB any 5* I recomend you to watch this video, there are good tips for LB