If you are thinking about development in a professional capacity, then starting with C# or Java makes strategic sense. You can start with a pace as slow or fast as desired; the time investment will be worth it. There are many good books (some free at http://mindview.net/Books/DownloadSites) and courses to assist with these powerful object-oriented languages. With your background, you might consider starting with PHP instead. For online tutorials, check my links below...
If you are a hobbyist, then Python or Ruby should be considered. During the 70's and 80's, the answer would have been Basic (primarily Microsoft's popular version). There are many flavors of Basic today such as VB.net, TrueBasic, Dark Basic, or Microsoft Office VBA. Some truly swear by Delphi (based on Pascal) but it has a rather limited audience.
On the cheap, you can simply start with JavaScript built into every good browser. JavaScript is very easy to learn and has webpage utility for DHTML interactions but will likely breed bad programming habits. However, the coding experience and constructs can translate fairly well to Java programming. I should mention that shell coding on Linux or Unix boxes will also provide programming experience that translates to other languages (procedural types).
If you have the time, select a couple languages to dabble in. Each tool has their own strengths and weaknesses. Therefore, there is no "best" language to master. IBM once thought they could create the ultimate general purpose language by mixing Cobol, Fortran, and Algol together; it was a mild success for their mainframes but didn't spread any further.
I would recommend you avoid functional, logic/list-based or rule-based languages initially. They can be easy to learn but the experience won't translate well to other languages. Therefore, avoid Haskel, Prolog, Lisp, Scheme, etc. There are hundreds of special purpose languages that won't help during transitions. It has its place, but I wouldn't recommend you start with a stack-oriented language (e.g. Forth).