I think Usopp is the coolest member of the Strawhats. He provides some good ol' comic relief, is one of the best developed, most three-dimensional characters, and he's had some really awesome moments. He's also a remarkably good sniper, despite being held back by his insecurities.
Usopp is very much aware of being one of the weakest and most pathetic Strawhats and this upsets him greatly. In spite of this, he puts on a happy face for the sake of his Nakama. He only really confides in Nami, since he feels that the stronger Mugiwara wouldn't understand how he feels. Zoro recognizes this, but does not mention anything as it would likely embarrass Usopp and hurt his pride. Instead, he subtly encourages Usopp to become more manly and confident on his own. Usopp lacks the fearlessness of his crewmates (to say the least) but he still stands up and fights for his friends, which IMO is far more admirable than someone who knows they can't lose. Usopp's backstory is also very heart-wrenching and well-written and his simple dreams of meeting up with his father and following in his footsteps as a great warrior of the sea are among the most genuine and moving of the Strawhats' ambitions. Post-timeskip, Usopp has matured a great deal and it's really awesome to see that kind of character advancement.
...And THAT is an unpopular opinion backed up with logic. I'm sure that not everyone thinks Usopp is the coolest Strawhat, but at least now they can see why I do.