What about Rayleigh he seems to be extremely intelligent which is shown by how quickly he was able to deduce the situation when we first saw him in Saboady and I would think he would be like an adviser to Roger aswell when he was alive since Roger seems to be quite like Luffy where as Rayleigh would be the smart one who would stop any quarreling between crew members as shown in Buggy's flashbacks and stop his captain from doing anything to crazy without a plan or at least making one quickly before something bad happens so I would put Rayleigh at least above Nami and Kuro and maybe Beckman but I'm not to sure about that since he does seems like one of the smartest characters if not the smartest but who's to say Rayleigh isn't aswell but for East Blue I would agree Benn is the smartest and probably Kuro and then Nami after that.
Also I would think Vegapunk would be one of the most intelligent characters aswell since he created such amazing inventions such as the Pacifista's and even artificial devil fruits. Also don't forget geniuses like Sengoku the brains of the marines and Dragon the man behind the revolutionary army who would have to be highly intelligent to be able to pull of what he has against the World Government.But overall Benn has a good chance to be the most intelligent considering he is the first mate and adviser to an extremely powerful Yonko and is quite strong himself which would probably be backed up by his quick thinking and great analyzes skills or Shanks would have had alot more trouble getting to where he is right now although there are other people who could be smarter so we'll have to wait and see more of what he's like first and some other characters we haven't seen the full extent of their intelligence from so I can't make a final decision until then.
Also I agree most of the characters are quite simple minded but if I had to chose a few of the worst I would say probably Luffy but it's really his care-free personality that makes him seem stupid but he is known to act before he thinks alot and not listen to what other people have to say before doing something crazy. Also there is Jango and Gedatsu, Jango since I mean who is stupid enough to hypnotize himself every time he tries it on someone else and sometimes failing to even hypnotize the original target anyway. Gedatsu is even worse like when he forgets to do things like talk, blink and even breathe making him known for his careless and forgetful personality which definitely takes away from his menacing appearance making him a big contender for the One Piece character with the lowest IQ.
Although there are several other characters who are just as, if not more stupid so it's hard to decide with so many of them but there the ones that stand out and also there is Zoro with his constant ability to get lost and has be seen to have quite a few stupid moments but as we've seen when it comes down to it he can get really serious and alot more tactical just like Luffy but even more so and since he corrects Luffy quite a bit when he's being stupid and can get very annoyed at him for it which shows a reasonable gap in there intelligence. You could also say Chopper has quite a low intelligence for believing Usopp's and falling for alot of obvious things but he is a doctor and apart from those moments is quite an intelligent character so I wouldn't consider one of the characters with a low IQ because that and probably one of the 3 smartest Straw Hats along with Robin and Nami and if he tries to be he is definitely one of the more intelligent characters in the show.
Overall it's hard to say who really has the highest and lowest intelligence but I would definitely agree Benn is a main contender aswell as some of the other ones I mentioned for having the highest intelligence and the others I mentioned for having lower intelligence would be up there when deciding on who would have the lowest mainly Jango and Gedatsu but of course you would need to also consider people like Wapol with his his generally stupid personality, Spandam with his clumsiness and several others but there are probably people with higher and lower intelligence out there that we haven't meet yet and especially when it comes to higher intelligence we still haven't seen many already introduced characters full potential so for now there are many characters that could hold that spot until we see more.
Although as I said before I do agree with what you say for East Blue Benn, Kuro then Nami but then again I could see Shanks being one of the highest at least in East blue I mean just because he has Benn doesn't mean he can't be smart so I might put him ahead of Nami but I am not really sure if he could have been or not so for now I'll agree with you Gobee.
And sorry for such a long post I just didn't know how to put it all into one short paragraph so I did six