I am just an intermediate whistler of sorts-- I am sure many more accomplished have already given advice here. I just thought I would add a few comments to elaborate on some already made. It seems to me that any brand of whistle requires a certain "touch" or "feel" (or whatever you want to call it) to bring out the best sound from it. Some whistles seem to me to be easier to play than others. That doesn't mean that the ones that are more difficult sound bad-- it just takes a bit more time and effort to develop the feel for them.
The first whistle I spent much time with was an Oak. After playing a number of others, I realize this wasn't easiest one to begin with. Oaks and Acorns require a light touch, especially moving between octaves. They seem to be much stronger as you move into the upper octave. On the downside, they always seem about 20 cents flat on C and C#. When played well, I love the tone of an Oak-- it seems to have a clear, ringing tone that I prefer sometimes. I have an Oak in D and C, and a couple of Acorns I generally use as "car" whistles.
I also like the Feadog. It seems to me to be easy to play. The intonation seems to be generally better on them. Personally, I like the tone they produce also-- slightly "chiffier" than the Oak, yet similar (possibly since Oak supposedly uses bodies made by Feadog?). I have a Jerry Freeman Mellow Dog, which is excellent- I especially like it with the "C" body. I have a Susato that was easy for me to play at first, but now that I have a little more experience, it is difficult for me to play and get pleasing sound out of, and I hardly ever try it anymore. I have a few of Tony Dixon's whistles (adjustable polymer D, aluminum D, polymer C, polymer A). Their intonation isn't perfect, but for me, they are some of the easier whistles to play (and get good sound from). I often pick up one of these when I'm just messing around at home.
I also have a few of Clarke's whistles (Sweetone, Original, Meg D and C). I'm not a big fan of the feel of the wood fipple in the Original, although it seems to sound pretty good. They all seem a little weaker (quieter?) in the lower octave to me, and get a bit stronger as you go higher with them. I have to be in the mood for their chiffier sound. The only whistle I have ever had that I believe has some sort of manufacturing defect is a Walton's Little Black. It almost seems to make a vibrating, uneven sound as you blow through it that I cannot make go away. I have another one of them that doesn't do that, so I am not sure what is wrong with it.
I really can't speak to high-end whistles. I haven't reached the point yet where I would feel justified in spending enough to cover one of Michael Burke's whistles. I have heard nothing but good from those who have played them. But, I believe you can get at least good, if not great sound from almost any whistle. It just takes some work, and development of the proper feel for the whistle to get the best from it (a good ear helps a lot, too). I figure that with a number of them, you can choose which sound you prefer for a given time, session, etc.
Thanks for reading my humble opinion!