That's really a difficult question to ask, and there's no single answer. Which language to learn depends greatly on what your intended goals are, and what you're good at.
C is a great language. If you are comfortable in C, you can get comfortable in anything. I use it quite a bit, myself.
That being said, Java or C# are useful if you intend to write bigger applications or want to learn object oriented programming. They're the most common languages used in enterprises, in my experience, and are useful that way. I don't use these much, though.
If you're more interested in the Web/HTML stuff, then Ruby on Rails, PHP, or even C# are good things to learn. Ruby has become incredibly popular lately, and it seems to be a nice language. I don't personally know Ruby, but I use PHP on occasion.
And finally, if you want to write quick stuff or you need to automate tasks, you should learn a language that I would call a "scripting" language, like Perl, Python, PHP, Ruby, Lua, etc. These are usually quick and simple, but also powerful. The vast majority of work I do is in languages like this, in my field.
Hope that helps!