I do not know what you mean by 'scripting language,' so I'll take it to mean 'a language with garbage collection, rapid development, and general ease of use.'
PERL, Python, LUA, PHP etc.
Perl is a good language for short programs that need to chop text around, and it's a trippy language to learn, too. Python is a somewhat decent language, big on batteries but small on power. (I'm such a language bigot, aren't I?) PHP is utter crap. Well, I'm being a bit mean about PHP, since people use it to do useful things, but it's just about the most poorly designed language that is frequently used today. It's basically a retarded version of C++, and that's only a bit more powerful and versatile than assembly language.
Lua is an excellent programming language, and of the four you have mentioned, I recommend it the most. The reasons are simple: it's simple, and it's got closures, first class functions, and proper tail calls. But this is still not my recommendation...the most powerful and versatile scripting language.
It is quite simple: every other language you will see in this thread has a set of features that is a subset of the features of Common Lisp. I recommend learning Lisp for a few reasons:
1. It's the most powerful and versatile language*. (Or maybe some other variant of Lisp is; this is just standardized and the most popular.)
2. If you learn Lisp, you'll never have trouble learning another language.
3. Your ability to think about programming will improve, too.
Don't balk when you see the parentheses. I can assure you they're not a problem :-)
*O'Camlers, Haskellers, and Prologers may feel free to disagree.