If you're looking for wireless? Nothing. IMO this is as good as it gets for bluetooth. If you're cool with a cable you can definitely get something better at the price point.
I would agree...but also for the price point you do have wired capabilities. I personally think the wired M2(not bluetooth version) sound different than the wireless version of the M2. The wired version can be had pretty reasonable and are great for their price point. I prefer the sound of the wired version but now only own the wireless for the ease of no wires.