The griffin ones work well, dont get battery powered because 10 mins into the use of it the battery power is too weak to create the ideal FM broadcast. I hate FM transmitters but unless i spend $200 on a deck/installation I pretty much have to deal with it.
From my experiences, Griffon is the way to go. But you really don't want to cheap out and get a $10 or $20 one cuz they'll sound way worse than your normal radio stations. Mine was I think $45 and sounds even better than the stations I get. The quality almost entirely depends on your head unit though. If you have a shitty HU that can't pick up FM signals for shit, you could buy a $200 FM transmitter and it'll sound no better than the $10 one a Rite Aid.