C: Overcomplicated, but fast, and useful for extending most other languages
Java: Reasonable, but a bit wordy. Pretty marketable. Performs quite reasonably really, though slow to exec
C++:Probably best avoided unless you have libraries in C++ you must use (in which case you could use Cython)
Objective-C: Useless, except on an iPhone
C#: Another lockin trojan horse from Microsoft. Avoid.
PHP: Awful design. Avoid.
BASIC: Those whom the gods would destroy, first they teach BASIC.
Python: Very nice language to work in. Sacrifices performance a bit in the reference implementation, but pretty fast if you use Pypy. Can be extended using C or Cython. Sometimes off-putting to people who feel that programming "should be complicated".
Perl:What a mess! It's much too kitchen-sink. For people whose problem domains aren't complicated enough to keep them entertained, so they need a messy language to make things more interesting.
Ruby:Popular among Java programmers who don't want to use anything but Java. Kinda perlish, but not quite as bad.