I personally like Chrom with Robin more, but Cynthia as Lucina's sister isn't bad. I recommend Chrom has Aether before you get Cynthia though. She can get the skill inherited, but she cannot get it naturally like Chrom or Lucina.
I might change my mind its a hard debate between Robin and Chrom and Chrom and Sumia on one hand Sumia works so hard on the other, Robin and Chrom have the most screentime uh bye the bye I've only gotten to chapter seven.