Obviously Horford. I am the one who started the Theis or Thompson thread before Theis thoroughly began outplaying TT, and I was still super pro-Theis in that thread. OKC acquired assets to take on Horford. I hardly imagine we would need to give up anything to get him, but it would be important that we stay under the tax.
I should note that I don't want anything to affect us re-signing Theis this off-season. I doubt he will break MLE level money and his versatility is just too important to what the Cs do.