What is best brain for rex?

As an AI language model, I can interpret your query in two ways.

1. If you are asking about "best brain" in general for the dinosaur species Rex, here is some information:

• T-Rex or Tyrannosaurus Rex is considered to have one of the largest brain sizes among dinosaurs.
• It had a brain size of about 80 grams (comparatively smaller than modern mammals like elephants and whales).
• Its brain was mainly designed for hunting and scavenging purposes with keen senses (smell and vision) and good cognitive capabilities to track prey and navigate the environment.
• Recent research suggests that it might have also had some social and communicative abilities, which are usually associated with animals with a larger brain.

2. If you are asking about the "best brain" in the context of humans selectively breeding the dinosaur for intelligence, then the answer is purely speculative and fictional. There is no scientific evidence or research about Rex's ability for intelligence intervention through breeding.