The edition of Milton's work I've selected for my students to use is The Complete Poetry and Essential Prose of John Milton (edited by William Kerrigan, John Rumrich, Stephen M. Fallon; Modern Library edition, 2009). Follow this link to Amazon where you can buy this for about $25.
Why pay for it?
Why buy an edition of Milton when all his writings are available online for free? Some of those online editions are very well done, like those found in The John Milton Reading Room at Dartmouth. Besides, Milton's works are readily borrowed freely from libraries.
Milton was all about liberty, and I think he would be pleased to have his works available for free. But he was also concerned with the quality of his publications. A good edition is trustworthy. It has been established from reliable sources, and in this case, the spelling has been modernized. And of course a good scholarly edition of a literary text includes an introduction, explanatory notes, and a bibliography. It also helps to have a common edition when studying with others.
