I say Oonoki. A's not touching Oonoki mainly because A cannot fly. Although Oonoki may throw out his back in which case I say Gaara wins.
I think Mei would be blitzed by Raikage, zapped by Jinton, or punched by Tsunade (loses to Oonoki, Tsunade, A)
Tsunade doesn't really have a defense to Jinton. She'd need to get close to Gaara. So (She loses against Gaara and Oonoki.)
Gaara could be beaten with Jinton. (Loses to Oonoki)
A would lose to Oonoki's Jinton, Tsunade in a fight, Gaara's Sand (which has stopped him mid-attack before). Though I suppose if Mei catches him with a Suiton that may pose problems for raiton cloak. (Loses to Oonoki, Gaara, Tsunade)