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.
