Best for mostFolger Shakespeare Library · 1992
For almost everyone, the Folger is the right Julius Caesar — facing-page glosses and scene-by-scene synopses make the rhetoric and the politics easy to follow. Cheap, and the classroom standard (this is the Shakespeare most American students meet first). This is where I'd start.
Most scholarlyArden (3rd series) · 1998
The Arden Third Series, edited by David Daniell, is the scholar's Julius Caesar — a long introduction and dense footnotes on the text, its Roman sources, and its afterlife. Indispensable for study; more than a first read needs.
For beginnersNo Fear Shakespeare · 2003
No Fear sets the original beside a plain modern paraphrase — a crutch, not the poetry, but a way through if the language has stopped you before.
William ShakespeareJulius Caesartr. Free (public domain)
FreeFree (public domain) · 1623
Julius Caesar is public domain, so the full text is free — Folger Digital Texts, MIT, Gutenberg. No glosses, but the whole play at no cost.