I like expressing my opinion and lucky me I just happen to have reasons for each esper.

Belias- In my first file I gave Belias to Vaan, the second I gave Balias to Balthier. Remember how Balthier went along with the payment of the treasure that lies in Raithwall's Tomb. The treasure ended up being Belias so I thought of a couple of possibilities for the situation. First would be that it would be a repeat of what happened at a previous situation you might recognise. Here's how it would go:
Balthier: Is that so? Call me old-fashioned, but I was hoping for a treasure
whose worth we COULD measure.
Vaan: So I can have it then?
Fran: Are you mad?
The other way I was thinking would be that Vaan would have to give Balthier any treasure he found for life in exchange for Belias. And I assumed Vaan would be suckered in.
Mateus- Bah, I don't think I can top that one. I gave Mateus to Ashe because I felt bad that she had to give away Belias as a treasure. Besides, Mateus has to do with Raithwall too.
Adrammelech- I believe I gave to Vaan because Vaan's quickenings were kind of thunder related.
Zalera- lol, you're not shallow, I've heard worse. I gave Zalera to Basch because he had a twin and Zalera is Gemini.
Zeromus- You're right, it would be a better esper for Balthier. I gave Zeromus to Basch because of the Judge thing.
Shemhazai- same as you I gave him to Fran for the same reasons
Hashmal- Ashe for the order reason but I wanted to give him to Basch for the same reason but I wanted the summons a balanced two to a character, except for Zodiark.
Famfrit- same as you again, to Balthier for the same reasons
Exodus- same again, to Fran because she's my favorite character of XII and Exodus is my zodiac. And the tree thing too.
Chuchulainn- To Penelo because I made up this thing where Chuchulainn liked Penelo (weird, I know XD), hey it's a reason you know.
Chaos- To Vaan because Chaos seemed to belong to the (althought debated you do get to follow the story through his eyes) main character. I'm still confused about the main character thing myself, but no matter.
Ultima-Penelo for reasons of balancing the field to two espers (cept Zodiark)
Zodiark- Who to give it to? I balanced it so whoever I gave this one two would not only have the most powerful summon but also have more summons than the rest. Turns out I didn't count Belias in the two per character thing because you HAD to have him to open that gate. So I gave Zodiark to Balthier so it could be balanced.
You know what would have made these decisions easier? If the characters had birthdays. That way, we could figure their zodiac.
Belugn, thank you for making this thread, I had fun sharing. =)