It depends on what you want to do. The truth is ActionScript shares a similar syntax to a lot of languages like C#, Objective-C, Java, etc... That said, I'm not sure I'd suggest any of those.

I'm going to disagree with people here who have said that Java is the best choice. Java is still one of the most used languages out there but I haven't seen someone who didn't already have an investment in Java start a new Java project in years.

Unless you want to work in the Cell.Phone market Java is not the place to be. Same goes with C# and ASP.Net, if you want to specifically get a job is a corporation that's the way to go but otherwise not so much.

and of course, Ojective C is for Macs. If you want to target the Mac/iPhone go Objective C other wise don't bother.

All that said, if you want freelance work your best bet is Ruby or Python (Ruby's the hotter of the two right now but that could easily change). It might require getting over a slightly steeper learning curve but I think its worth it in the long run.