Unfortunately, due to the very low output power restriction they have, the best type to look for is one where you can set your own frequency, rather than from a fixed selection. For example, the transmitter built in my GPS can be set to any frequency between 87.5MHz and 108.0MHz in 0.1MHz steps:
From my experience of using a handful, they are only effective when set on a frequency where you would just hear static. For example, if you live in an area where you hear a station anywhere you tune your radio dial to, then it's unlikely any FM transmitter will get give a clear signal.
In our family car, we ended up replacing the radio with one that has a line-in, as the car had the FM aerial at the very back which meant that the FM transmitter had to be in the back seat for the aerial to pick it up properly.
Another thing you could try is moving the FM transmitter about the car to see where the best reception is, then run a headphone extension lead to the transmitter.