Toragi and Seer are top chaps. You do not make a mistake bringing them to 60.
Melga and Bellower are also very useful, Siegehulk can deal great damage.
Spirithost is useful in the early stages of arena, Gnarlhorn aso has it uses, but both do not need to be 60...
By the time you have Toragi and Seer (plus your starter champ ) at 60, you will have more options to choose from
You should also consider running some potion dungeons to ascend your champions...
Seer early would be wrong due to just not being able to gear her properly, nor having the buffs necessary.
I would go Athel, then Siegehulk. Siege is fairly underrated and gives us a 2 turn defense on a 3 turn cooldown, and can hit hard when built right.
After that it would be between Melga or Toragi for me. Toragi should help you in Dragon and Clan Boss immediately (and forever) when built out with a lot of HP/DEF/ACC, so I don't mind going him.