Lisp

I'm new to programming, and I've been learning Python and Fortran, the latter because it's part of my mathematics degree. I want to learn Lisp because it supposedly makes you a better programmer. Which dialect should I learn?

Scheme, simply because Structure and Interpretation of Computer Programs is such a mind-blowing text. SICP introduces computer science as a new way to formally and concretely express and prove imperative knowledge of "how to do something", complimentary to mathematics which provides the language to formally and concretely express and prove declarative knowledge of "what is true". Scheme, whether because or despite its simplicity, turns out to be an extremely powerful language for expressing these ideas. By the time you learn how easy it is to take partial derivatives of polynomials in Scheme using its native syntax, you start to get the idea that it's superior to even mathematical notation, which is the basis of Structure and Interpretation of Classical Mechanics, an advanced mechanics book taught in Scheme.

That's not to say Scheme is the perfect programming language. Modern programming requires a larger set of native data types than Scheme provides, and functional programming in general isn't suited to very large programs. But if you're picking up Lisp to become a better programmer, SICP will do the trick, and that just happens to be taught in Scheme.