Wow, um, a lot of "C/C++" votes here. That's pretty funny, considering c and c++ aren't very expressive. If speed were the only factor to consider, c would win, but then, why go with c? Why not go with assembly?
Programming language choices these days aren't about speed. Programming languages are for designing programs. A well designed algorithm will run fine in any language you spin it in (unless it lacks the libraries to accomplish the task!) The language is just a matter of choice or convention. Modern programming languages exist to help the coder write better software. Software runs on very very powerful hardware these days, and it's a shame a lot of programmers are still living in the dark ages of imperative c and c++ code. Overhead is acceptable if that means the code is more manageable.
Haskell, in my opinion, is the best language. Or at least one of the best I've ever seen. It's not perfect, nothing is, haskell is a work of art. It's a purely functional programming language. The expressiveness of Haskell is killer, most stuff can be done in a few lines of code. The separation between type and function allows one to design software by defining the types and then building functions to work on the types. Monads encapsulate unpredictable elements, such as code that interacts with the system or values with states. The functional style allows code to be broken up into little discrete functions that make code modular and composable. The syntax is very clean and distilled. It's a beast language, and it's just about as fast as C++.
Haskell isn't popular probably because the learning curve is extremely steep. You learn some basic stuff, like variable and functions. But then you find out all the variables can't be changed and even worse to introduce state you have to understand some thing called a monad. There's a lot of theory in Haskell to get through before you can start really designing good stuff. That's a huge downside, but at the same time it means Haskell coders are freakin' smart.
As far as making games in haskell goes, that's even trickier, because systems like games, with tons of state and events flying around, require knowledge functional reactive programming. It's really, really powerful. Basically you build these systems that react to changes in some data. Great for something like a video game. Still, that means writing a game in Haskell requires very advanced knowledge, so it's not a very common thing.