An obvious choice is the classic Thunderer 558 (sample)
For something a bit smaller and shinier, there's the Thunderer 60.5 (sample)
Or how about a replica of the whistle they used on the Titanic, the Thunderer Titanic (sample)
The ACME Survival whistle is the most similar to the bog standard Perry Whistle I currently carry.
The ACME Signal Horn isn't exactly a pocket whistle, but wow! Me want. (sample).
The Tug Boat Whistle is another totally impractical one, but what a great sound. (sample)
The Siren Whistle sounds absolutely incredible, but I can't tell from the picture if it's actually small enough to carry about on your keyring (I suspect not). (sample)
ACME's first whistle, the Metropolitan Police Whistle still sounds good after 130 years in production and looks rather shiny too (sample)
But the ultimate has to be the ACME Meteor. Slightly impractical though, as blowing it involves two skydivers sitting on it and jumping out of a plane.
Edit: No, I'm not getting a commission for writing this. Maybe I should ask for a discount ;)