I guess it depends on what you mean by mobile (and what you budget is).
I personally call
Emotiva's Airmotion 4S portable because I can fit two speakers in a backpack and since they are powered I don't need an amp with them. These are pretty good speakers, especially for the price at $300/pr.
Otherwise if you are looking for something smaller; there's the
Emotiva 3b at $170 or the
Audioengine A2+ at $250. The 3b's include bluetooth and the A2+ includes USB audio however, given that both of these are smaller the bass isn't as deep as the 4S.