Best for mostFolger Shakespeare Library · 1993
For almost everyone, the Folger is the right Twelfth Night — facing-page glosses keep the jokes and the cross-dressing plot clear, with scene-by-scene synopses. Cheap, and the classroom standard. This is where I'd start.
Most scholarlyArden (3rd series) · 2008
The Arden Third Series, edited by Keir Elam, is the scholar's Twelfth Night — a long introduction and dense footnotes on the text and its festive world. 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 ShakespeareTwelfth Nighttr. Free (public domain)
FreeFree (public domain) · 1623
Twelfth Night is public domain, so the full text is free — Folger Digital Texts, MIT, Gutenberg. No glosses, but the whole play at no cost.