'Simple to learn' is always going to be relative - the biggest burden is making the jump from imperative to functional programming, the syntax is of much less importance. In fact, a radically different syntax might make it easier to pick up the new concepts and approach programming from a completely different angle.
The language itself is probably less important than its surrounding ecosystem - you want to pick one that has an active community, a rich set of libraries, and most of all, good development tools. However, I'd go for a language that was designed for functional programming, rather than a multi-paradigm one that happens to be suitable for functional programming - javascript, for example, has many FP aspects, but it doesn't invite or urge you to use them.
Here's a list of languages you might want to try:
- Haskell
- Scheme
- Common Lisp (CL)
- ML
- F# (if you're into .NET)
- Clojure (if you're familiar with the Java ecosystem)
Of these, Haskell is probably the most radically different from what you're used to.