As much as I dislike poll threads, I must add to the discussion. Warning, as this has spoilers! This is assuming you have read up to the most recent chapters!!!
Franky is the oldest crew member, but that does not mean he is the most intelligent, but he has his own…"unique" flair. He knows enough about boat engineering to create his own boat designs from scratch; even as a child he just took scrap junk and made workable boats out of them, probably even without a blueprint to begin with. He even knows about various things relavent to the shipwright business, like the Adam the Treasure tree, whose wood can absorb water and reduce damage from the seas. Thanks to Tom and Iceburg, he learned how to hone his skills. On another not, being abandoned as a child, he has a sense of protecting others from being the same, so he acts as the "big brother" figure, whether it be the for the ex-bums of Water 7 and making him his Franky Family, or his new family of the Strawhats. His technological expertise goes so far as to become the first "cyborg," as more than half of his body is made of metal and prosthetics, yet can function as weapons as well. Even though he should have DIED when he was hit by the Puffing Tom, his will to live and seek revenge for his foster-father allowed him to survive and make himself into a human battleship, powered by carbonation like a steamboat is powered by steam. Like a ship that protects the crew from the harsh seas, Franky wishes to protect his family from the harsh world, like Tom did before him.
Robin, being older than most of the crew may be more knowledgable, is not the most intelligent. She know more of the world they live in from memory, but that does not mean she can solve problems quickly or create complex plans. She is the wisest, as she has a a wide array of history and facts of One Piece. If anything, if someone is not sure about the World Government, the Grand Line, or a mystery in general, Robin will be the one to go to to clear it up.
Chopper, although only 15 and naive to the outside world, has the knowledge of a 180 year old doctor (Dr. Kureha, if you recall). He has been trained ever since he was little and to have that much knowledge at such a young age, you have to be a genius with incredible recollective memory. However, his knowledge is extremely limited to the medical profession…we all know how silly he gets when Luffy or Usopp impress him with menial things and lies.
Sanji, while still a girl-crazy fool, has his moments of quick wit and heroic acts. He has a knack for planning things out in advance in the shortest amount of time. For example, he concocted the break-in and entering of Rain Dinners to save the crew from Crocodile by using Chopper as a distraction and causing a panic in the casino in almost no time flat. He even used the perfume he gave Nami to make Chopper track her down if they got lost WAY earlier in Alabasta, just when they got there! In Skypiea, he managed to stall time for Usopp and Luffy to rescue Nami by breaking the insides of the Maxim. And most recently, he used the closing of the doors of the Gate of Justice to create an escape route for the Going Merry, as closing them caused the whirlpools and messed up the aim of the feared battleships. If anything, Sanji is a master of sabotage and chivalry (saving the crew out of tough situations). Like a cook can make you or break you depending on how he prepares his food, Sanji prepares the crew to safety while spoiling the plans of others.
Usopp is smart because of his ability to create the almost impossible weapons. His artistic ability is a facet of his way to "lie" to his enemies, as his weapons hide their power in meek looking forms. He weilds a slingshot, a toy of a child, but can hit from afar with deadly pinpoint accuracy with a multitude of ammunition. To be able to be that flexible to shoot anything from a slingshot is amazing in any respect, as one needs to account for weight, aerodynamics, power of the pull and strength of the slingshot bands to fling them as far as he does. His newest weapon, Kabuto, is also a feat of engineering because he can use the staff part to stick into the ground for stability, and more bands to increase the distance and power of any shot, with a larger bag to hold more ammunition per shot. Kabuto is like a mix between a sniper rifle and a shotgun, depending on the range. And yet, he still uses his original slingshot as a pistol, and has a variety of weapons in his bag, making him like Felix the Cat and his own magical bag, aiding him in any situation.
Nami probably has the best intuition and common sense of the crew, even though her common sense is often dismissed by others and makes her upset. Nami, as we all probably know by now, can draw maps with technical accuracy and a lot of time by eye, but she may rely on tools to aid her. Her sharp eyesight, feelings, and knowledge of the weather is an invaluable asset to the crew, as she can predict storms occuring just seconds away from just senses alone! She must have the most sensitive feeling of air pressure to realize the subtle changes in the atmosphere in order to do such a feat. Also, she probably has the best sense of balance of the whole crew, which is essential to have when manning a boat, as noted by her immidiate use of a Waver in Skypeia. We have seen Nami come up with off-the-cuff ideas like leading Luffy to Enel with the Waver tracks, as well as negotiate a bigger payout from their gold haul in Water 7 thanks to her almost unparalleled desire for money, she knows the value of material things. Combined with 10+ years of studying weather patterns, currents, and islands of East Blue with Arlong, she managed to take that knowledge and become one of the most cabable seafarers of the One Piece World. Ben Beckman, as Oda has pointed out, is probably smarter than Nami by years of experience, but that may change soon enough.
Zoro's knowledge is limited only in his sword-fighting, as we have seen he has a variety of moves in using 1, 2 or 3 swords. However, being so focused in his sword practice, he never had a proper education, thus explaining his non-existant sense of direction (a VERY important thing to know in a world covered mostly with ocean and limited technology). Being as old as he is, he probably does not care to learn anytime soon, but sooner or later he will figure that this will present a huge problem in the years to come. He is the rock to Luffy's insanity more than Nami, because he is trusted to be the first mate and is actually quite sensitive to the feelings of the crew more than anyone despite his hard exterior. He knows the captain has to be strong because "if he falters, he cannot be fit to lead," as noted from the fight with Usopp. His strong, hardened philosophy is mainly from his promise to be the world's greatest swordsman, because in his dream, there is no room for failure; he is either the world's greatest or not, and not means death.
Finally, Luffy is quite possibly the most IDIOTIC person in the crew, but he has his moments where he shines brightly as the captain, and this is usually concerning trying to save his friends and acquiring new ones. His stupidity is possibly a side-effect of his grandfather always concerned into making him into a Marine, combined with the family narcoleptic trait that he, Ace, and Garp share (and probably out of silliness, Dragon may have it too), making him incabable from being awake while being taught anything. Combined with an attention span of a 10-year-old, his childishness and immaturity almost knows no bounds. I say almost because he has moments of maturity, as he would risk his life for them. He believes a pirate is the definition of conviction and power against those who are close-minded, but a pirate is nothing without his crew most of all.
So….that is my whole take on this.