What makes Philip Larkin one of the greatest poets in the English language?

Larkin outside the university library where he worked in Hull. 

Larkin was not given to blowing his own trumpet (nor listening to ones played by the 'sour' Miles Davis). He was doggedly self-deprecatory, referring to himself as 
 “the dude/ Who lets the girl down before/ The hero arrives, the chap/ Who’s yellow and keeps the store.” Since his death, however, Larkin has been increasingly been recognised as the preeminent poet of his generation - heading  a recent Times poll of the best (post-1945) British writers 


But what makes for literary greatness. According to Martin Amis there are two key qualifications: memorability and originality. I would add a third: humour.


He married a woman to stop her getting away
Now she’s there all day,
And the money he gets for wasting his life on work
She takes as her perk
To pay for the kiddies’ clobber and the drier
And the electric fire ...


From  'Self's the Man' The Whitsun Weddings  


Michael Dirda gives an excellent short introduction to Larkin's work here.