Big fan of spider, but needs to be booked.  Great aura for dungeons, and great aoe skills.  In arena def down/weaken against all affinities.  Tough to gear because low base speed.  Dont wory about atk/cr/cd, go speed acc def/hp res.  

Frozen Banshee is who I would 6 star for Clan Boss.

1-FB

2-Spider

3-Fahrakin  the  Fat

4-Catacomb  Councilor

Armiger  will  also  be  a  priority  to  six  star  eventually  further  in  to  dungeons.  The  first  two  can  interchange  depending  on  what  you  want.  For  Clan  Boss  and  Dragon,  FB  would  be  the  choice.  For  general  dungeon/arena,  itd  be  Spider,  assuming  you  would  be  able  to  book  them.  I  like  focusing  on  Clan  Boss  because  it  empowers  you  to  work  on  other  areas  with  the  rewards  you  get.  There  are  others  you  coyld  six  star  too,  such  as  Balthus  and  Warcaster.  It  all  come  down  to  dependence.