My keychain whistles are the flat FOX jobs. Hardly perfect, but I bought a fistful on clearance, they're very durable, and they're not bulky.

(The ACR has been favourably mentioned here before. I haven't had a chance to test one -- I guess I'll need to scrounge one online. They're not in the stores up here.)

The second I go afield, I upgrade to the FOX Sharx. These dudes are LOUD, and not unmanageably large. The somewhat lower frequency travels farther IME. My only gripe is the fact that the blast is directed sideways instead of forward. If I had to toot one of these for a long time, I would be half deaf -- I wouldn't notice the SAR helicopter hovering above except for the wash.

The gold star goes to the Storms, which always go in key kits, but they're so darn big. The sound projects forward a bit more, I find. It's also friendly to lower-powered blasts, which brings back the hounds without making my ears ring more than necessary.