like many ppl have stated if you want to do high damage in PVE then go for Ikkaku if your planing on using Kisuke + Rukia. and if you want a good PVP partner then Byakuya is your better pick as he is a good mix for both PVP and PVE until lvl 90 when you can get Zaraki
Just my honest Opinion though so its up to you on what you want to make out of your formation at the end of the day
I would say Ikkaku first, then use ikkaku power in PVE to get Kisuke for extra fury. If your planning on Ikkaku, then you might as well need to have Rukia, Kisuke because they are overpower in PVE. If want PVP, i reconmend Byakuya.