Contrary to other responses: A hyphen is required in "second-largest" if being used as a compound adjective, as in: "That is the second-largest pizza I have ever seen."

Is it second largest or second largest?

thank you in advance. No. Both 'second largest' and 'second-largest' are used. 'second largest' is used about twice as often as 'second-largest'.

Is second best hyphenated?

Second-best is usually hyphenated when used as an adjective.