I think good IEMs will provide the best audio, but they may not work for everybody.
The first problem is – there are no quiet helmets. Some are less noisy than others. So helmet speakers no matter how good or correctly located will always have to overcome ambient helmet noise. As speed increases helmet noise increases. As noise increases the rider has to increase speaker volume. Long exposure to moderately loud volume levels can be damaging over time.
I have a Sena 20S and use the helmet speakers. I've added the various pads to bring the speakers closer to my ears for better sounding audio. However I notice whenever exiting the freeway and slowing down the music gets much louder.
Obviously that's not correct. As speed decreases so does helmet noise. Suddenly I'm hearing how loud the music has been.
I'll be trying IEMs at some point. A tad less convenient than helmet speakers maybe but better sound overall when on the road.