all 3 are valid options. I do think this is a personal preference sort of decision.

ask yourself what device(s) you want to connect. ask yourself how much you want to spend.

if looking to save $ and assuming your head unit aux in is a 1/8" trs (had phone jack), and assuming your drive has the same, this is a very easy cheap option.

if $ is not an issue or you don't have a device with any out put connector or you mention blutooth and want arguably the most convenient option, the switching out the head unit is the way to go...

in this day and age I think the FM transmitter is an antiquated and least convenient method.

not sure anyone could give u an answer here... just their own opinions.