I've been holding out OCaml because it didn't have good concurrency support, but that seems to have changed recently. I've thoroughly enjoyed learning Haskell, and I'm sure ML has a few treasures of its own to give.
OCaml has had great concurrency support for quite a long time, the Lwt promises library offers async I/O and works really well. Now, with OCaml 5 shipping Multicore support, even more performance will be unlocked, potentially (it really depends on the design and workload characteristics of the application).