Looking for the LOUDEST rear wheel / hub.

Chris Kings on my bikes seem to work for peds on shared paths and such,  they always seem to hear it and react, it's an unusual sound. Backpedaling a little makes it louder yet. Apart from that, a polite 'excuse me'or 'on your right' doesn't hurt either. 

Plus 1 for Hunt with the latest hubs. So loud it was embarrassing wheeling the bike through our office foyer every day. I sold them!

Campag hubs with a Campag spline are noisy too, but nothing like as loud or insistent as the Hunts. 

Turbospoke nah, the continuous noise will drive you nuts. Way back in the day, these are what the "cool" kids had :-).

More seriously though, might be worth considering something like a Timber! Mountain Bike Trail Bell

My Hope RS4 hubs are some of the loudest I've heard and I have yet to hear one louder out on my rides!

Sounds like you need some spokey dokeys. Used to come free in packs of cereal back when i was a kid..

https://www.amazon.co.uk/Bicycle-Spokey-Dokey-Spoke-Beads/dp/B004CXZSDA

I would never condone an Air Zound

(but a friend of mine finds them hilar.. very safe)

Second vote for turbospoke, or one of these

Go for Hunts with their Sprint hub. Crazy loud. 

what about a few zipties on the seatstays and leave the ends long to tap against the spokes?

Hope hubs or playing cards and a peg

Vero hubs are quieq noisy, but nothing like Hope hubs

Don't go for Hunts, I have them and they sound like Shimanos but a little bit angrier maybe. Try Hope. I bought a Whyte MTB a few years ago and the rear wheel was so loud I had a similar replacement built with an XT hub and sold the Hope on, it was RIDICULOUS.

Lolly stick and a couple of elastic bands.

DT Swiss freewheels are pretty loud.

Try a Turbospoke.

My Vision Team 35s are loud, as are most of the Hunt wheels. Generally the louder wheels are the ones with the fewest degrees of engagement (more/stronger pawls) so anything that's at the racy end of the spectrum is worth considering.

I bought a second hand tandem that came with Hope hubs. Here’s a cheaper option if you are good with your hands...

https://m.youtube.com/watch?v=ZMXC284JrSU

Campag. Especially if low on grease.

Chris King hubs are reportedly very loud, big on the angry bees, the woods are often alive with the sound of Hope hubs.

The new Shimano XTR is apparently silent.  Mmmmm..

 I do not have an answer nor suggestion for a loud hub.  And, yeah, those rinky dinky ding-ding bells are for kids toys. On my Fixie, I solved my pedestrian problem with a mini air horn that blasts out a 120 db toot - 1 mile audible distance.  Gets cars/trucks to move out of the bike lane too. Picture below is basically what I have. Taped the horn to the clip and slide the clip over my right side belt/waistline.  Measures 4 inch high x 1.5 inch diameter. Co$t around $6.00 US. Lasts a long time.