Best is prooobably his mother. She's dead in canon, so he wouldn't be stuck having to deal with coming to terms with the uneasy feeling of "She died." He's already done that! Instead he'd just get to work on learning how to relate to her as a son to a mother and building up the relationship and getting to protect the shit out of her. It still isn't an ideal situation to regain her in, but it is the most positive reunion with a person he could get in this place.

Worst isss a tough one. Comedy answer is Bianchi for obvious reasons, but more seriously it's probably Byakuran as the last terrible enemy he remembers. The one that scared him enough to go to Sabra in the first place.

What would Zexion's feelings be if Dera did start to remember camp?