I'd go for F#. It teaches you to think in a very different way and can improve your programming habits and style as well as your ability to reason about algorithmic problems.

I'm not sure if it's an easy to learn outside of an academic setting, but here we're learning Scheme as our first year language and I think it's a very good choice. There's little syntactic mess, it can be reasoned about with mathematics (which makes it easier for a number of mathies that don't have prior programming experience) and it's even possible to jump into concepts like interpreters after only 1-2 month. F# is nice but type inference is too complicated for a beginner.