I'm not gonna lie -- Amelia Reacher is my #1 pick for companions.
You meet her somewhat early in the game when you need to take a rover out to visit a far away site. She becomes your companion a bit later, after some crazy stuff goes down.
She's the third companion available for romance.
Amelia doesn't have an inner filter. She speaks her mind and doesn't care how you feel about it. She's pretty angry with you when she first joins the team, but over time you gain her trust back.
Amelia is a Guardian, so I tend to give her the best mace and shield I can find/craft. She does a pretty good job of it too -- she does decent damage but, like David Ward, is best for basically just distracting a few enemies.
Amelia gives Resistance to Distruption +5%. Not bad, considering how important disruption is in this game.
Once you earn the Ally bonus, you get a +1 Crafting. I think this is one of the best Ally bonuses, as almost every player will do some crafting to upgrade their gear.
Amelia is similar to Scott in that she's involved in several quests. The main ones directly helping her are about repairing the rover and a quest involving Scott and her dad.