When a fictional world is consumed in a zombie apocalypse situation, intelligence is often measured by just how well the characters adapt for survival. It's necessary that they be observant, quick thinking in high stress situations, and creative in their approach to problem solving. Resident Evil: Welcome To Raccoon City is no exception to that idea.
Some of the main characters in the movie adapt quickly, thinking through plans and making decisions at the drop of a hat. Others, however, need a little prodding. Some of that is down to shock, but a lot of it is also a result of not thinking things through. Most of the smartest characters survive to make it out of Raccoon City.
Leon S. Kennedy
Leon isn't unintelligent, but nearly anyone else in the movie makes better decisions than he does about the state of Raccoon City. It's clear that he's not entirely suited to his role as a police officer as he isn't the most observant of characters.
Leon doesn't pay enough attention to what's happening outside of the police station to notice that he should close the gates outside of the building until it's almost too late. He also isn't observant enough to clock that one of the men in the holding cell of the station is rapidly changing into a zombie-like creature. At least Leon has the presence of mind to get the train running and grab the rocket launcher to kill the biggest monster in Raccoon City.
Chris Redfield
Chris Redfield isn't quite as unobservant as Leon, but he certainly willfully blocks out certain things. He doesn't see Dr. Birkin for who he really is, for example, until it's too late.
Chris also ends up pinned by virus-infested locals and wouldn't have escaped without the sacrifice of one of his teammates. Though he is one of the veteran members of the team, he doesn't think all of his actions through in the movie. That leads to him being the one with the most injuries as the group attempts to escape the city.
Chief Brian Irons
Chief Iron is the man responsible for keeping Raccoon City put together, and while the officers working under him might not know the whole truth about the Umbrella Corporation, he clearly does. As soon as things start to go south, he tries to get himself out of town.
Irons is also the only one left in the police station by the time the violence starts who knows a way out. His plans are pretty well thought out as knows about the underground tunnels and the more populated areas of the city left. The trouble is that he, much like Leon, isn't quite as observant as he should be. If he was, a monster wouldn't catch him completely unawares.
Albert Wesker
Wesker might not be the most likable person in Raccoon City, but he's not unintelligent. He sees an opportunity to get out of Raccoon City, and he takes it.
Even though Wesker isn't entirely sure what's going on, and he isn't given all of the details about what the Umbrella Corporation really does, he still manages to follow the cryptic clues sent to him by Ada Wong and follow the maps through an unfamiliar location. Wesker almost makes it out of the mansion on his own, but Dr. Birkin is too determined that no one else will get his research to allow for that.
Jill Valentine
Jill doesn't get a huge amount of screen time in Welcome To Raccoon City, but what the audience does see of her demonstrates that she is incredibly clever. She outsmarts Wesker when he makes a bet with her at the start of the movie, showing that she's got some creative problem solving skills.
Jill is also the only one to notice that the helicopter they left behind is heading straight for the mansion. She manages to save her and Wesker's life, though he does go on to betray her. Jill doesn't leave her other friends behind, able to navigate her way to Chris Redfield despite Wesker having been the one with the maps. She proves herself a survivor.
Claire Redfield
For a lot of fans, Claire Redfield becomes the best character in the movie simply because she's the one the audience spends the most time with, and she's the one the most detailed by the writers. Claire also, however, happens to be one of the smartest people trying to survive the events in Raccoon City.
When the audience first meets her, she doesn't seem to be making great decisions as she's hitchhiking with someone who runs a woman over, but she's the first person to voice that something's wrong in the town. She's able to follow Ben's train of thought with his conspiracy theories, pick her brother's unpickable lock, and save the lives of Leon and Irons as soon as she meets them. Claire also knows that something strange is going on with Umbrella as a child, demonstrating good instincts when she trusts Lisa more than she trusts the adults around her. She's easily the most observant one in town, and the one who is the best at putting puzzle pieces together.
Dr. William Birkin
Dr. Birkin is, ultimately, the real villain of the movie, not the Raccoon City residents who have unwittingly been poisoned by the Umbrella Corporation. Just because he's the bad guy, however, doesn't mean he isn't also brilliant. Birkin is easily the smartest of the main characters in the movie.
It's Birkin's experiments that fuel this arm of the Umbrella Corporation. From the research he was doing when Claire and Chris were children to prolonging his own life by turning himself into a monster, Birkin's experiments are unethical, but they show the audience the brilliant scientific mind underneath. Birkin is responsible for the entire chain of events that sets the Resident Evil franchise in motion; his mind creates all of the monsters.