By Popularity, unfortunately soundwave >>>> Blaster.

In actual battles, there have never been a time without outside influence where they can duke it out one on one with their cassettes either.

Their function differs a bit don't they? Blaster is more of a communication expert, while Soundwave is meant for disruption and interception of messages, etc.

I like to vote for underdogs like Blaster (who, although functions the same as Soundwave, was introduced later and thus lost the "first cassette intro'ed" awesomeness.