I think no one has answered this yet because we may get into a complicated discussion! Anyway, I will risk it. The only one which is wrong is "London is the city where I visited last year." This sentence is the answer to: "Which city did you visit last year?" (not "where" or "which place") If this is not enough for you to understand the grammar here, please feel free to ask.

Hello Satoshi, Michael's answer is good but there is also a third possibility London/Paris is the city I visited last year. If the relative pronoun (which/that in this case) refers to something which is the object (after the verb) of the next clause then you can omit the relative pronoun. London/Paris is the city I visited ... I visited London/Paris last year. Hope this helps Bob