Depends on what you mean by "best". Sound quality wise in descending order IMO:
Aux
Bluetooth
Wired FM modulator
Cassette adapter
Wireless FM transmitter
I'd replace the head unit before using either of the last 2 options but, again, your priorities may differ from mine.
If quality isn't your only concern then the list may take on a completely different order for you. USB is only relevant if you want to listen to music on your SD card. Several aftermarket companies make aux adapters for factory car stereos. You'd have to check their application guides (usually on their sites) to see if any of them work with your make/model. DICE, Blitzsafe, Peripheral Electronics, and USA Spec are just a few companies that I can think of off the top of my head.