A general overview of the consensus of critical opinion, which seems to be rather uniform, is that John Milton's pastoral elegy Lycidas is a masterpiece for Milton and the most masterful example of the genre of pastoral elegy. An article by Mark Womack starts out by quoting some of the prestigious critical comments made about Lycidas. He reports that Lycidas has been called:
"the high-water mark of English Posey"
"probably the most perfect piece of pure literature in existence"
"the greatest short poem of any author in English"
"the touchstone of poetical taste"
"the most perfect poem of its length in the English language"
For references to these opinions quoted by Womack, consult his article available at Questia.com [they offer a free 24-hour trial period if you haven't got an account.]
Lycidas is a marvel of a poem because it defies the constraints of its genre and form by deftly incorporating elements from other genres and forms. For instance, Milton used an irregular rhythm and rhyme that is a defining characteristic of the polyphonic Italian canzone form. This has great significance because Lycidas is a monody for one voice (polyphonic is a form for more than one voice). Another illustration of Milton's expansion of Lycidas through characteristics not relegated to pastoral elegy is the introduction of extra speakers with personas, a technique not used in single-speaker (monody) elegy but rather a characteristic of the epic genre.
In addition to these sparks of genre-stretching genius, Milton incorporates a metaphoric conceit for Christian salvation and redemption into the characteristic shepherd related language of the pastoral through Biblical allusions to Jesus as the Shepherd. Further, through a significant digression beginning at "The Pilot of the Galilean lake..." Milton makes a profound comment about the Clergy of his day and accuses them of being imitation shepherds (Biblical allusion) who can't even hold a "Sheep-hook". These are but some of the reasons for the uniform high praise of Milton's Lycidas.
[For more information, see Lycidas, Milton Reading Room, Dartmouth College and "The Value of 'Lycidas'" by Mark Womack.]