Who should be my lead unit?

Elise

Use Summer Robin, Summer Elise, Halloween Nowi, or Bridal Caeda to bait other people in.

Definitely Nowi.
With Summoner support, ally support, & blessing,
Lightning Breath+Res Fury QR Nowi can took down almost everything not green. She can even took out BLyn & Reinhardt in the same turn given those two didn't have any buff.