Who is the servant in Isaiah 53?
The nation of Israel. http://outreachjudaism.org/gods-suffering-servant-isaiah-53/
As per that website, that has been the position of Jews for a long time
Origen, a prominent and influential church father, conceded in the year 248 CE – eight centuries before Rashi was born – that the consensus among the Jews in his time was that Isaiah 53 “bore reference to the whole [Jewish] people, regarded as one individual, and as being in a state of dispersion and suffering, in order that many proselytes might be gained, on account of the dispersion of the Jews among numerous heathen nations.”
Origen, Contra Celsum, Chadwick, Henry; Cambridge Press, book 1, chapter 55, page 50
Some other resources http://www.aish.com/sp/ph/Isaiah_53_The_Suffering_Servant.html
A longer discussion
Some other stuff http://www.whatjewsbelieve.org/prooftext10is53.html
and this entire chapter dissects the quotes:
sorry for the loose links, but there is too much to quote.
The answer is: the people of Israel, and specifically the righteous remnant of Israel . See Abravanel, Alshich, Even Ezra, Malbim, Metzudos Dovid, Metzudos Tzion, Rashi, Radak, Ramban, Shadal, etc...
A detailed article by Prof. Uri Yosef's on this can be found online here.
For further reading: Isaiah 53: Who is the Servant? by Gerald Sigal