I just got the UE Boom. A little expensive, but it has good sound quality and volume. I want to buy a second one, because two can be used at the same time for stereo sound.
I know you said portable, but I thought I would share my setup. I came up with the idea for a Bluetooth speaker a few weeks ago. I was looking at $80 speakers, then it occurred to me that I could just make my preexisting studio monitors Bluetooth. After all, they have great sound quality, and I already had them, so I went ahead and bought a Bluetooth receiver. I have friends over all the time, so I went ahead and made a NFC tag that connects the speakers. With the tag attached to my qi charger, I can just set my phone down and it will start playing my music.