i do not think you can go wrong with any of the above suggestions.
However, since 5* Heroes take 353,599 Hero XP and 15.3m food to level, I would wait until I had enough ascension items to level two of each color and see what I had in my roster then.
if I had to pick I would go Hel because she is cool or Kunchen because you can never have too many healers. With Proteus and Rigard, neither Hel or Kuchen is a role you do not have a hero to fill. Pick which makes you happy.
Isarnia because Grimm 4* 4.70 has the same defense stat as Gunnar 3* 3.50 . And my teammates say Isarnia goes well with Zimkitha.
Lianna because you will eventually have multiple green, but Lianna as a ranger sniper can do insane damage.
Joon because even dead Joon’s tile/ matching/ physical damage is very high ( grave damage). The recent buff to Justice makes this the toughest choice.