box2D is a bit bloated for javascript at the moment, but depending on what you want to archieve, it shouldn´t be too hard to write your own perfectly suited physics which runs fast enough.
I really hate the unity web player and flash is also dead to me since there is the possibility to do the same without plugin and probably faster in at least some aspects.
The only big problem with webgl is that it doesn´t run iOS. But for a relatively simple 2D game inline svg could work pretty well on any platform with HTML5.

For simple 2D, I would just write everything from scratch myself, as everything else is usually too much.