Best for mostFolger Shakespeare Library · 1992
For almost everyone, the Folger is the right Merchant of Venice — facing-page glosses and synopses, and a good edition matters here because the play's treatment of Shylock needs the context the notes supply. Cheap, and the classroom standard. This is where I'd start.
Most scholarlyArden (3rd series) · 2010
The Arden Third Series, edited by John Drakakis, is the scholar's Merchant — a long introduction that takes the play's antisemitism seriously, with dense footnotes. 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 ShakespeareThe Merchant of Venicetr. Free (public domain)
FreeFree (public domain) · 1600
The Merchant of Venice is public domain, so the full text is free — Folger Digital Texts, MIT, Gutenberg. No glosses, but the whole play at no cost.