My old favourite language is Common LISP. If you have a mathematical mind, the initially disturbing syntax seems second-nature after not so long. While I've drifted away from it in the last few years, I still return frequently. There are many implementations, and CLISP and SBCL have always worked for me.
A few years ago, Python changed programming for me. I recommend it strongly to beginners. While the performance is not optimal, I don't tend to complain: if I felt that every nanosecond counted, I'd be writing in some other language anyway. However, not enough people are aware of Cython and Psyco. The latter, in particular, is unintrusive and offer a massive performance boost for almost no cost in time and effort.
Most of my programming nowadays is involved with the fledgling programming language Fortress. It's designed to be good for doing mathematical computations on parallel systems. It's great fun, and Sun Microsystems don't seem to mind me getting involved.