Any variant of the 470 or 480. Apple chose to collaborate with AMD on the lowest performing GPU possible, the 460 ... which is a low end budget GPU. You can get the desktop version for 120 bucks. A 120 dollar GPU in a 2600 dollar machine. The 2300 dollar machine starts with an even worse GPU in the 450, which is half as powerful. It actually costs 200 dollars to upgrade to the 120 dollar 460. Apple uses a severely gimped PSU at 80 ish watts ... and the chassis limitations from their never ending pursuit of thinness has probably led to their complete inability to put anything better in their machines. It has become abundantly clear that putting any sort of decent GPU in their laptops is not ever going to happen. But this isn't just true for their laptops. It's also true for the iMac. They've made the iMac so thin that heat dissipation is terrible. They use mid-tier mobile GPUs even in the 5K iMac with no option to upgrade to anything more powerful.
If at this point nobody sees a problem with this, then I'm sorry ... I don't know what to tell you. The fact that people are okay with any of this really shows how deeply entrenched people are. People are defending a budget GPU in a super premium priced device like their life depends on it. It is not in any way logical.