Byakuya. ByakuyaByakuyaByakuya. Renji's life is suffering, so let's make it more stressful.

Or, on the other hand, there is Kira Izuru, Hisagi Shuuhei, Madarame Ikkaku, Ayasegawa Yumichika, or Ishida Uryuu.

OR. On the terribly obscure end, Renji's biggest fanboy, Rikichi!

Finally, there's the Terrible Options of Aizen Sousuke or Szayel Aporro Granz.

YES, I THINK THAT'S A PRETTY GOOD POOL.