I mostly use the fox 40 mini or the dolfin, which sound reasonably similar to one another. (Since I like the dolfin, I'm pretty sure I'd like the Valkeen, too, but I'm just not willing to pay that for a whistle.)
When fields are next to one another, I usually use either the Fox 40 Pearl or the Acme Tornado that looks like a bend piece of gum--because they have a more unusual sound. But I think they are both harder to "talk" with than the mini or the dolfin.
I have a handful of others that I use from time to time. I like the newer Fox 40 Fusion better than the classic (but neither as much as the mini). I've used the Sonic Blast and thunderer, but never particularly liked them. I had a devastator (that's the one with a spin wheel inside instead of a ball), but was luke warm on it--I lost it at a tournament and never got another one. I also still have a metal whistle with a ball in it from when I first reffed as a youth, but I never use it.
And I have a few others, but since I haven't had my ref bag out since February, I don't remember what else is in there.