Best for mostPenguin (1855 first edition) · 1855
Many readers and critics think the book that matters most is the original 1855 Leaves of Grass — leaner, wilder, and more radical than the later versions Whitman kept revising. Penguin prints that first edition with Malcolm Cowley's classic introduction. The best way to meet Whitman at full voltage. This is where I'd start.
The complete finalDeathbed edition (1891–92) · 1892
If you want the whole of Whitman's life's work in the final form he authorized, you want the 1891–92 'Deathbed' edition — everything, arranged as he wished it. More complete, less concentrated than the 1855; the completist's choice.
Walt WhitmanLeaves of Grasstr. Free (public domain)
FreeFree (public domain) · 1892
All editions of Leaves of Grass are public domain and free online. Free and complete — though for the 1855 text with context, Penguin's edition is worth the few dollars.