I like Common Lisp's ability to annotate functions with types *for performance*. It removes much of the dynamic typing overhead.
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Type signatures are optional in Haskell, but in OCaml you can even specify partial signatures (e.g. just one parameter).
Maybe return types are more valuable to annotate? I can often guess params based on the name, but I have to read the function body for the result.
Emacs tip of the day: annotate your functions! The byte compiler can replace entire function calls at compile time if your function is pure.
Rather than showing compiler errors in a different window to your editor, BSD has long been able to annotate code! https://lambda-the-ultimate.org/node/5280#comment-90779