Editions

The standard scholarly edition of Johnson's works is the Yale Edition. It's been in progress for about four decades now; the following volumes are available:

Vol. I, Diaries, Prayers, Annals, ed. E. L. McAdam, Jr., with Donald and Mary Hyde (1959);
Vol. II, The Idler and The Adventurer, ed. W. J. Bate, J. M. Bullitt, and L. F. Powell (1963);
Vols. III, IV, V, The Rambler, ed. W. J. Bate and Albrecht B. Strauss (1969);
Vol. VI, Poems, ed. E. L. McAdam, Jr., with George Milne (1964);
Vols. VII, VIII, Johnson on Shakespeare, ed. Arthur Sherbo (1968);
Vol. IX, A Journey to the Western Islands of Scotland, ed. Mary Lascelles (1971);
Vol. X, Political Writings, ed. Donald J. Greene (1977);
Vol. XIV, Sermons, ed. Jean H. Hagstrum and James Gray (1978);
Vol. XV, A Voyage to Abyssinia, ed. Joel J. Gold (1985);
Vol. XVI, Rasselas and Other Tales, ed. Gwin J. Kolb (1990).
Vol. XVII, A Commentary on Mr. Pope's Principles of Morality, Or Essay on Man (A Translation from the French), ed. O M Brack, Jr.
Vol. XVIII, Johnson on the English Language, ed. Gwin J. Kolb and Robert DeMaria, Jr.

The edition isn't yet complete, and given their pace, don't hold your breath. For the rest of the major works, these usually serve as the standard editions:

Lives of the Poets, ed. G. B. Hill, 3 vols. (Oxford: Clarendon Press, 1905), was long the standard edition; a new edition, ed. Roger Lonsdale, 4 vols. (Oxford: Oxford Univ. Press, 2006), promises to supersede it;
The Life of Savage can be had either in Hill's Lives or in the more accurate text edited by Clarence Tracy (Oxford: Clarendon Press, 1971);
The Dictionary is available in several large facsimiles of the first edition (1755) and the fourth (1773); they're all, I'm afraid, expensive. Anne McDermott, though, has recently edited the full text of both editions on one CD-ROM, available from Cambridge Univ. Press for around $300. See also Anthologies & Selections.
Many of the minor works are available in Samuel Johnson's Prefaces and Dedications, ed. Allen T. Hazen (New Haven: Yale Univ. Press, 1937).
What's left just hasn't received respectable scholarly treatment in this century. If you don't want to go back to originals, you'll have to go back to either the 1787 edition of the Works edited by Sir John Hawkins, or the 1825 Oxford Works (reproduced in 1970 by AMS Press).

There are also three modern editions of Johnson's letters; the best and most recent is The Letters of Samuel Johnson, ed. Bruce Redford, 5 vols. (Princeton: Princeton Univ. Press, 1992-94), but R. W. Chapman's edition (Oxford: Clarendon, 1952) is still handy, especially for numbering the letters — a system Redford dropped.

Some other useful scholarly editions, even though not "standard": The Poems of Samuel Johnson, ed. D. Nichol Smith and E. L. McAdam, Jr. (1974); Samuel Johnson: The Complete English Poems, ed. J. D. Fleeman (New Haven: Yale Univ. Press, 1971); A Journey to the Western Islands of Scotland, ed. J. D. Fleeman (Oxford: Clarendon Press, 1985). The diaries, prayers, and annals are available (with other works) in Johnsonian Miscellanies, ed. G. B. Hill, 2 vols. (Oxford: Clarendon Press, 1897).

So much for scholarship. But most of the volumes above start at around $50 each and go up from there. (One exception: Liberty Fund has reprinted the Yale edition of the Political Writings in a paperback for around ten bucks.) If you want to own cheaper editions, Fleeman's edition of the English Poems is available in paperback. Bronson includes a reliable edition of Rasselas in his paperback Samuel Johnson: Rasselas, Poems, and Selected Prose, 3rd ed. (San Francisco: Holt, Rinehart, 1971). Both Penguin and Oxford do inexpensive editions of Rasselas. For just about everything else, you'll have to make do with the Anthologies & Selections.

This is part of a Guide to Samuel Johnson by Jack Lynch. Comments are welcome.