The best translation, hands down, is this Modern Library Classics by Bloch and Bloch. The layout is very user-friendly, with the original Hebrew on one side, with the English translation on the other. Anyone who knows or is studying Hebrew will appreciate this feature. Also the Song jumps between voices and narration without the typical "he said"/"she said" obtrusions that are common to modern writing. Using context clues it is not too challenging to figure out if the lines belong to the Lover or the Beloved, but the translators' use of italics and bold, makes this distinction clear and renders a very smooth reading.
What makes this translation supreme is Bloch and Bloch's great command of Biblical Hebrew combined with their artistry in maintaining the poetic beauty of this text. They took great care to translate this text as closely as possible to the original Hebrew, but still made some artistic adjustments when a too literal translation would be awkward.
This edition includes a wonderful introduction which discusses the various historical interpretations of this text, as well as their own take. A detailed commentary follows which explains in minutiae the justifications for various translation decisions. For example the word "love" in 5:1, Bloch and Bloch argue has in Hebrew really the specific meaning of "love-making" as opposed to other Biblical terms for love such as the NT Greek "philos," or "agape," or the OT Hebrew "chesed" all of which can have meanings quite different from the act of coitus implied in "love-making." But the Blochs are careful to point out that the same Hebrew word used for love in 5:1 is used elsewhere in the Hebrew Bible, and in those instances is very clearly in the context of romantic lovers spending the night together.
The Bloch translation manages to capture the mystery, the beauty, the sensuality of the language, in a way that is fresh, readable, and true to the Hebrew.