From those listed there, the 3 I'd pick for you are:

Meryl
Leina
Ceilim

Do note, however, that both Leina and Ceilim are about to get nerfed because they are currently considered very strong in pvp. How they turn out afterwards will be seen later.
Leina is also a 2 star, so might be hard to get rank ups for unless you dedicate a lot of resources, including diamonds, to her. But as of right now, she is well worth it.

Khalis is about to get a buff that sounds significant, so we'll see after it happens. Can replace Leina if desired, but will be even harder to rank up, and currently is completely outclassed by pre-nerf Leina in everything.
Dora can replace Ceilim if the nerf bat hits her too hard. Can also still be worth raising Dora, for the fact that she is a "ranger" class and will use a different set of gear/upgrade materials. Meryl/Leina will use the same set, so gearing will be slow when sharing loot.
OR: Replace Meryl with Elnora.