The Mpow Streambot Y is the best FM transmitter for 2015, according to The Wirecutter. CNET also recommends the Mpow Streambot Y along with the Satechi Sound-Fly View Bluetooth FM transmitter, giving both a rating of 3.5 stars out of 5.
The Mpow Streambot Y does not have automatic tuning that finds a frequency that gives the clearest sound. The Wirecutter recomends it for those predominantly looking for an FM transmitter for music.
CNET notes that the absence of a 3.5mm jack requires users of the Mpow to have a device with Bluetooth capability. It also notes that the Satechi has more options for audio sources, including an SD card slot, 3.5mm jack and Bluetooth.