It depends on what you want to do with it.
For instance, I'm a 3D Game designer/programmer, and I mainly use a compination of C/C++ and Assembler. (Assembler for optimization. All trig routines, vector calculations etc, are in here)
For compiling I use Gnu C/C++ because of the portability. I work in windows, dos and Linux.
Considering this is in the web design forum, I am guessing you mean what web based programming language is the best to learn?
Well, it all depends really .. as you have probably befuddled (figured out) by now .. the main programming languages in use on the web are:
Perl/CGI
PHP
ASP
CFM
JSP (which I doubt you will be using so I won't summarize it.)
Now, lets sum up each language ..
Perl
Okay, the grand daddy of modern web languages .. it was the language of choice in its day, and the UBB (the software that used to run this forum) uses Perl very efficiently, but these days it is getting harder to write good complex Perl scripts without either killing the server .. or being able to do it better with less resource usage in PHP. It is still useful for some things though, especially if you are using flat text files.
PHP
Well, what can I say .. great conditionals, excellent database functions, good string commands .. a great language! It is the language that runs these here forums and you can see how powerful they are! PHP is absolutely the #1 choice if you are interacting with any sort of SQL database (postgreSQL, mySQL, mSQL, SQL). My language of choice at the moment.
ASP
An excellent language (sadly only still on Windows servers (mostly .. can anyone say chillisoft asp?)). A great language, especially for anyone who can already code in VB then ASP will be a god send to you, it has good database functions especially for Access databases.
CFM
ColdFusion Markup .. well, in its most basic form it is a hyped up version of HTML .. but very hyped up .. it is sort of like PHP/HTML/JavaScript in one which makes it an excellent platform, but it does have its flaws and sadly isn't widely supported yet.
Okay, there ya go .. an extremely quick summary of each language, for me learning PHP was the easiest (but I had learnt Perl first). But I was able to code proficiently and efficiently in PHP after about a month, this was a month of spending about 30 mins on it a day .. so I would opt for that if you were learning a new language.
For learning, I'd say go for PHP. Its fairly simple to pickup and has excellent built in support for databases. I've picked it up in about an hour.
{ Granted, I have over 18 years of programming experience; started with Basic and Assembly, moved onto Action! [Atari 8Bit language] and Pascal, then moved onto the more popular languages -- C in 1991, C++ in 1993, Perl in 1994, Java in 1997. There's a few additional languages mixed in there too, like Prolog/Logo/Lisp/Ada and others I won't admit to knowing... }
My personal choice for development is JSP/Java-Servlets/EJBs since I have the great tools to do so from where I work at.
Perl might be a bit essoteric to learn, depending on what your learning source is. So give PHP a try!