FM transmitter choice, I've learned, is based on personal preference more than anything else. FM transmitting technology has kind of reached as far as it's going to go, so they're all pretty much the same. I can, however, give you a geek's POV and preference list
After I lost my previous one, which had no charging power, I purchased this
transmitter which was similar. It just plugs into the headphone jack of ANY phone or MP3 player. It also has a USB output on the bottom for charging (an upgrade from my first one). It worked perfectly. I say "worked" because about a month in, my brother stole it for HIS car. He left it on his dash all summer (not generally a good idea in So California) and the sticker faceplate over the station dial came off and now the wires are slowly being exposed.
Overall, I would recommend it if you're just looking for something inexpensive that you might be able to use with other products with headphone jacks. Just take care of it (my brother never does...) and it should be a good choice; especially for $5 plus shipping!
Now, being a geek, I look for stuff that's unique and still very convenient to use. I originally found this product
during my search to replace the one my brother took. It says it's for the iPhone, but the hookups are likely to be good with any phone that has bluetooth.
Similar to that, is this product
. It's less expensive and also has the added bonus of a remote control that can mount to the steering wheel.
I hope that helps some.