Argh, don't wanna choose only one. My second choice would be web browser. Out of curiosity, you said fully supporting all the plugins on tons of platforms is unrealistic - what about partially supporting some? Like having only the rendering engine, sprites, sound and a few behaviors (and array and hash, I guess)?
Also, I'm sure you've already thought of it, but what about using something like allegro or SDL to make it easier to make both a mac and linux runtime instead of having to choose one?