I just use my lungs. I’m loud from my time in the army so shouting has always worked.

I use a small and simple bell, mainly because of the size.

130db-150db impact horn with remote.

When I was a kid, it was playing card rubberbanded to my chainstay so I could pretend I was riding a motorcycle from the noise

Nowadays, I prefer to just let out wolf whistle. Pierces even the best noise cancelling headphones and busiest traffic noise.

I like those classic ring ring one as it doesn't sounds aggressive just to tell people i'm approaching, but sadly i couldn't find it in market near me. On the other hand, i almost had a collision with someone who wear earphone and stare right into their phone while on their evening stroll, where after i ring my bell, he proceed to shift his direction onto my path. Have to move out of the way to not hit him, and he only see me after i went pass him.

With everyone using those god damn earpods now, I think you could literally have an air horn attached to your bike, and they still wouldn't hear it. I just have a little plastic bell attached to my bike, and I have about a 20% success rate with it. It always gives me pause when someone acts startled, and shocked, yet I've rang my bell 4 times, and my 108 tooth hub freewheeling is literally roaring like a swarm of bees. Yet it's naturally my fault I've startled them.

Air horn because it works against cars.

playing music publicly works and probably the only time it's not annoying.

I've got a little Knog bike bell. You press it once and it lets out a "ding" once. I'll press it two or three times if I see someone wearing headphones who won't hear the "On your LEFT" as I come up on them.

Bonus, I can put my thumb on the bell real easy and it shuts up.

It was cheap, does the job, and is really small.