If you have to go with an FM transmitter -- I've been there before, and again, it's my only option in my current car unless I get the head unit replaced -- get one manufactured by Aerielle Technologies. They have their own branded models, but they've also produced them for other companies as well. I had a Kensington Aerielle transmitter several years ago for an iPod, and it worked amazingly well.
I have a Dynex (from Best Buy) one made by Aerielle. It's got a 3.5mm audio jack and a USB port so you can charge at the same time, albeit you have to use the two separate cables to do both. The sound quality is phenomenal as far as transmitters are concerned. I can use pretty much any frequency I want; it blocks out all but the strongest broadcasts. Model number is DX-FMDC1. I'm pretty sure they've been discontinued, but you can get one on eBay for $10 or less. I was lucky enough to get mine for $5.