Idk honestly. My first 6 I just got recently and traded her for Doro (I lost a lil in value in my trade but give no *** as I originally wanted Dorothea anyways and she came at level 80). I think a great starting 6* would be a magic user since they are abundant at 6* level but not really it seems at other levels (must be a reason for that).
Magic dmg is prettttttty damn useful for trial style events (fortunately the very next event is trial style), and poison or AoE especially are great. Poison 6-7 is extremely potent on the right leader. Though I will admit, the short period I used Anna (she's a missile slow 6), I was actually impressed by her. Slow 7 with Battle Seasoned is a literal dead stop to whoever she hit. She was just too slow for my tastes. I realize Dorothea is a D speed as well, but she's poison 6 so her dmg is longer than just her initial hit, making her speed a not so much factor as her dmg carries on to whom she hits and I've seen many monsters die to that additional poison damage. I really like Dorothea as a starter 6. Just NEVER WASTE TE ON GETTING ALEXANDER for your first 6*. He's pretty damn sick in Arena (because he's slow but extremely high dmg and has a chance to AoE proc), but other than that, he's absolute trash.