which ONE ?

I read this sentence: "Which do you think is the most interesting?"
shouldn't it be:"Which do you thing is the more interesting one?". If not..why?! "I THINK LONDON IS THE MOST INTERESTING" or "I think London is the more interesting ONE" ?? Why in some sentence we have to put ONE ? thank you

"Which" already implies "one". :)

<em>Which (one) do you think is the most interesting (one, out of them)?</em>

English hates redundant words, so "one" is removed. We only include "one" if we really need it!