The question, as worded, needs some repair. “Best” implies some criteria that must be met—Most famous? “Deepest”? Most finely wrought in rhyme and scansion? etc. At base it is a value judgment based on subjective criteria—the poem that best expresses my personal feelings about love? His sonnets are the most often cited as Shakespeare’s “poems” (by which is meant “verse”), but he also wrote five longer poems, none of which get the attention of Hamlet, Macbeth, or Romeo and Juliet, but are nevertheless large, subtle, complex, important works. Of the sonnets, probably the most often quoted (if only partially) is Number 18 (“Shall I compare thee to a summer’s day?”), but all of them are read aloud every year on Shakespeare’s birthday in many cities throughout the globe. Of the longer verse pieces, Venus and Adonis (1194 lines) is the longest; “The Phoenix and the Turtle” (66 lines) is the shortest and the “rhymiest.” If taken as one poem, the Sonnets, it can be argued, are the best-crafted poems. Finally, since Shakespeare wrote his plays largely in verse, it can be argued that the “best” Shakespearean verses lie inside his plays as soliloquys, monologues, even preludes.
Sonnet #18 is one of my favorites:
Shall I compare thee to a Summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And Summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And oft' is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance or nature's changing course untrimm'd:
But thy eternal Summer shall not fade
Nor lose possession of that fair thou owest;
Nor shall Death brag thou wanderest in his shade,
When in eternal lines to time thou growest:
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.