There is no question that Edmund Spenser and William Shakespeare set the standards for sonnets. The sonnet form has a structure that is the most powerful of poetic forms because it unites so well structure with the development of theme.
Rhyme directs the ear and mind both. Edmund Spenser was assiduous in his use of rhyme, adhering to the grand poetic tradition of his predecessors, especially Chaucer. In his sonnets, the rhyme scheme is one that interlocks and musically flows: ABAB BCBC CDCD EE, whereas William Shakespeare follows the rhyme scheme of ABAB CDCD EFEF GG, which often builds his rhyme to the crescendo of the final couplet that hammers the argument built up in the quatrains. In addition, Shakespeare often employs syntactic parallellism with his rhetorical constructions that adds an emphasis not demonstrated by Spenser. For example, in Sonnet CXXII, the Bard writes with parallel phrases and words both,
All this the world knows; yet none knows well
To shun the heaven that leads men to hell.
Whereas Shakespeare presents thematic arguments with dramatic effect, Spenser is more of a storyteller and lyricist, moving in linear direction with less variety of rhyme. Shakespeare, on the other hand, unites meter, rhyme, figurative language and meter into the whole of his argument. The octave introduces an idea, followed by an altering direction or mood--this turn is called the volta--and then the concluding argument of the couplet, that pronounces "a truth" with a finality to the momentum of the sonnet.