Probably paredit. It's really so much better than non-structured editing. That said, there are structured editing plugins/modes/editors for other languages too.
However, looking back at Lisp/Clojure now I really don't like the syntax anymore. I don't care about the brackets, it's absolutely true that you learn to look past them in a matter of days, but the homogeneity and the "denselesness" really impact my ability to quickly parse an expression. Infix syntaxes make reading code much easier, imo.