This is very much a matter of personal taste and budget. My opinions of those I have:
Copeland - the best to my tastes and the most expensive. I second Jim’s experience here and the G is among my very favorite whistles, although I don’t play it as much as the D, but more than the A. These whistles have a particular pop and resonance that make them IMHO unique. They are beautiful and well-made. They take moderate air. Some say there are issues with inconsistency; I like all eight of mine.
Overton - I had both an A and G and both were terrific. They take less air but harder push and are known for their back pressure. They are about half the cost of Copelands, but may be an acquired taste.
O’Riordan Traveler (anodized aluminum)- currently available only second-hand; maybe about two-thirds the cost (new) of Copeland. They tend to cost more second-hand. I have the G which is so different from the Copeland and every bit as fantastic. Playability and transitions are effortless and the tone is uniquely angelic or velvety. Small tone holes.
Abell blackwood - I have the A; it’s very fine and a bit less than the Copeland in cost. It’s wood and has some air in the tone and a bit quieter than the others. Lovely tone. He makes them in Delrin as well; I have a soprano D in delrin that I like, but others don’t feel it has the richness of tone of the blackwood.
ALba - I have an A/G set; as I recall, somewhat less expensive than Overton. These have a good deal of air in the tone. Lovely tone, but I don’t find the sound as focused as the others.
Water Weasel - I have the A and it’s probably still best bang for the buck. PVC with fairly large tone holes and a big sound. Probably about one fourth the cost of a Copeland.
Susato - I have the Kildare G; it’s plastic, has smallish tone holes and I think costs about as much as the WW. It’s a real solid whistle and a good G. I’d take the WW between the two, though.
Sindt - I had the A at one time. I just found the tone not exciting and I like the D and C much better.
Burke - I had an old G in aluminum and found it again unexciting. However, I have several others, including the new black tip soprano D and C, and they are terrific. Burke whistles are models of consistency with pure full sound and easy playability. The new black tips add a stronger bottom end and some real pop with a crisp attack on the tone holes. They are perhaps the most comfortable whistles for me to play for long periods of time. If Michael’s newer models in A and G are anything like this, wow.
As you see, very much a matter of taste and budget. Everything above is made by makers who are well-known and respected and have their followings. They are all good to deal with (Susato I have only purchased from retailers, not from the maker).
You want a top notch experience with a special sound, budget be damned, get a Copeland. You want great but different and unique sound at less cost, get Overton. You want something that is reliably spot on, no worries every time, less cost still, get a Burke. You want to try a very solid A or G for the first time without breaking the bank, get a WW.
Hope that helps rather than confuses.
Best,
PhilO