If you think about it:
Number one must be the
JVC RC-550 just cause it was kind of the first real street warrior and its whole design was laid out to be played outside.
The looks of it are also very very unique in the boombox world.
Number two would have to be the
Conion C-100F because in my opinion it is the best looking boombox ever made. The clichee of a boombox is the C100 - you think boombox, you think C100. The GF777 might have been outstanding with 4 woofers but it was never the first thing i thought about when i wanted to get myself an old radio

.
Then number three can only be the infamous
JVC RC-M90. Not because of its looks or size (okay, it is pretty big and looks pretty much like a ghettoblaster as you would picture it) but because of its appearance on LL Cool J's cover and, most important, its sound. I think only few will disagree when i say the M90 is easily the best sounding boombox out there. And whoever disagrees has never heard one.

So while there are plenty of other boomboxes that are iconic and stand out, I think the top three are the ones named above