Difficult question, I think the poll should be expanded a little. Where are the greybeards, for example. I wouldn't put Tullius or Ulfric up.....Because I don't think either of them are the cleverest person in Skyrim, no matter what I think of them!
I voted Esbern, you get the feeling you could ask him about anything and he would give you an answer. Calcemo is smart and snotty, but beyond Dwemer and Snow elves I don't think he is exceptional. Mirabelle is a smart woman, but a brilliant mind? I wouldn't say so. I suppose Septimus is clever, but he is also dumb enough to get tangled up in the snare of Herma Mora, again, I think a little too mad to be considered smart. Elewen, I don't understand this option, she is clearly just the stooge of superiors higher up, basically just a Thalmor spy and threat - maker, rather than an independent thinker. Urag gro-Shub has responsibility of a library, that doesn't make him an automatic genius. He may be, it may be that he just doesn't want to show off, I suppose he knows what he is looking at when examining items, which is a sign of intelligence. I would consider this a reasonable choice.
Ah, Tolfdir, he is a close second to Esbern in my book, he is clearly a deep thinker and exceptionally talented in his field, but again, he lacks that broad-reaching knowledge of all things.
Personally, I would put Delphine up there. She may be hated, but she is also smart. It takes skill to survive the Thalmore for so long, and she is obviously an exceptional agent in her field. I have a respect for her, even if she does rather underestimate what her duties to the dragonborn entail.