HTML5 / CSS3
2 Major Advantages
1) It already fits in with skillsets that you have developed as a designer / web designer.
2) It is pretty new and will have a long shelf life for you.
Flash is switching over only to hardcore gaming and deep analytics video, both very interesting fields and a nice area for Flash to explore, and yet not as diverse as it once was.
Java is a pretty heavy language, and probably really best used on backend systems, databases and all the unpretty things under the surface of the web.
Having design, in particular UI/UX skills to mix and match with the new HTML5/CSS3 language set is going to be a very valuable skill and one that I think will benefit the right brain as much, if not more than the left brain thinker.