Twelfth Night (1599 or later) is the last of Shakespeare's happy comedies. After that, while he was writing his great tragedies, Shakespeare wrote problem comedies.
     --Definition of "problem comedy"
     --The problem comedies: All's Well That Ends Well, Measure for Measure and Troilus and Cressida

Hamlet is the first in the series of Shakespeare's four greatest tragedies. This Mt. Rushmore of Shakespeare's tragedies was carved in stone by A.C.Bradley.
These four have certain characteristics in common:
     --A focus on the fate of a single character. (As opposed to the double focus of Julius Caesar and Romeo and Juliet, and the surrounding issue of Rome or Verona.)
     --A focus on the inner turmoil of the divided self of their heroes.
     --Philosophic eloquence:

• Hamlet: "To be, or not to be: that is the question"
• Othello: "But there, where I have garner'd up my heart, / Where either I must live, or bear no life"
• Gloucester (King Lear): "As flies to wanton boys, are we to the gods. / They kill us for their sport"
• Macbeth: "it is a tale / Told by an idiot, full of sound and fury, / Signifying nothing"