
David.
Ash was just a dick. he didn't play well with others, and did he actually achieve anything at all?? If he did he kept it to himself, didn't save him.
David might be nuts, but at least he got things done.

Good question! David certainly comes across as pretty smart...I mean...he did all that genetic engineering and all. Plus, he quotes poetry all the time...that comes across as pretty intellectual.
But, I'm not convinced that the other synths aren't equally smart and just less show-offy about it.
Plus, it's really hard to define intelligence. Walter is not designed to be creative, and creativity can be considered part of intelligence.
David's emotional intelligence may be pretty low, because he doesn't seem to have a well developed capacity for empathy or impulse control.
Bishop recognized and admired the complexity of the xenomorph when he studied it, so I think his level of scientific understanding is likely similar to David's.
Plus he clearly has a higher emotional intelligence, and he's demonstrated a capacity for creative thinking (volunteering to crawl through the pipe and remote pilot the ship).
Also, when Walter and David got injured, they didn't have much of a reaction. Whereas Bishop's natural reaction to injury seems more conducive to survival overall, and how can we define intelligence if not as "possessing the skills necessary to survive"?
Bishop is definitely the most intelligent.

The madman, David-8. Ash is deceptive but the execution could be better lol David not only deceived the Prometheus crew, he deceived the Covenant crew, destroyed the Engineers on their home planet, and created the Xenomorph variant.
"The trick, William Potter, is not minding that it hurts."

David 8 is a true intellectual athlete with a stellar performance record. David 8 has improved since losing his mind. He busted a spring or two but was still able to use Engineer tech on the Engineers.

David is the smarter of the two, and the most devious imo.

Indeed David! He always got a plan and manipulates his way through. Bishop seems very smart too.