Tressa - "You got more than you bargained for!" is one of the best victory lines in any RPG ever. Finding random money lying around has been so useful in gearing up out of starter stuff. She's my only chararacter that covers 2 physical and a magical element so far, giving her the most versatility, and I'm saving tons of money through purchase and making tons of money from collect so I'm able to use the best things I've seen purchasable.
Ophelia - I can only imagine the amount of healing grapes I'd be going through if not for Ophelia. I can't believe you can't use Grapes out of combat and have to use the healer too, that adds a bit of additional challenge, and would make not grabbing healers early rather tricky I'd imagine. She's quite a potent damage dealer with Holy spells too.
Cyrus - I like his analytical skill that reveals vulnerabilities before I even need to take an action. It saves me some trial and error on exploiting weakness. Cyrus spends much of the beginning of battles just being kind of worthless, but once I've saved up some of my boost, I let loose with the big spells. I was going through too much SP early on just using his spells for breaking.
Olberic - Right now, Olberic is new, and I'm sure this spot will change. He has the AoE physical attacks which is good, but not much else to say about his abilities to warrant something higher on my list.