post two characters who are always bickering/arguing/fighting/competing and say whose side tu usually take.

kyo and yuki they always fight and i will take kyo side

Zoro VS Sanji from One Piece. I always take my sweet Zoro's side cause I amor him !!! <3

naruto vs Sasuke.
I usually take Naruto's side. Sasuke is willing to go over dead bodies to achieve his goals, naruto isn't like that.

Fai and Kurogane from Tsubasa Chronicle. Their arguments are más of Fai teasing Kurogane and Kurogane getting pissed off, but whatever. I always choose Fai's side, simply because that's exactly what I do to my friends XD

Shizuo and Izaya hate each other xD I'm on Shizuo's side cuz he's fucking awesome!~ :3

Allen Walker and Yuu Kanda from D Gray man. I usually take Allen's side but my cousin Lenalee takes Kanda's side and then WE start bickering and fighting. So sad.

France/Francis Bonnefoy and England/Arthur Kirkland. I take England's side because he was a bit más mature than the other one.

Tai and Matt in Digimon
In spite of Tai being reckless, I would take his side because Matt's criticizing antics would sometimes make Tai mad.

Natsu Dragneel and Gray Fullbuster

I'll probably take the non stripper and go with Natsu during the arguments, Natsu's side because HE DOESN'T STRIP and he's just plain energetic

Well, they both cool down a lot later, but earlier on in Blue Exorcist, Rin and Bon have a pretty intense rivalry. They're still rivals and argue from time to time, though.
I have to side with Bon. Simply because he isn't as stupid and he's one of my waifus. XD