The ACR seems interesting; I'd like to try one, since the form factor is very handy. I have a bunch of the Fox 40 minis and they're okay as backup whistles, but aren't quite as loud as I'd like, and I prefer a lower pitch. I carry a Storm when I really need to make noise.

I'm surprised at the good reviews of the JetScream. I bought one and it's not loud at all -- very disappointed. Maybe I got a dud?