Omaker bluetooth speaker
It gets pretty loud, the size is perfect for a cup holder, and the connectivity is great (30ft line-of-sight and still working!).  The bass and high-volume tones are distorted, but that's unavoidable for an inexpensive, small speaker.  For the money, it's a great speaker.  If you want something that's actually high-powered and excellent tones, you'll need to spend closer to $80-100 on a larger unit.