I have several Generations in various keys and I don't think they are as bad as some make them out to be. The Bb is probably the best. Like many whistles intonation above top G (in a D whistle) is not brilliant. I tend to use them for practice.

I have a Tony Dixon polymer D whistle which is tunable and with good intonation, but needs handling gently as it takes very little breath pressure to make it overblow especially in the bottom three notes. I also have Tony Dixon Alto G and A polymer whistles which I like. Similar pitch to a treble recorder and nice tone.

The Sweetones - well the less said the better. I don't like them - horrible things.

I have a Susato set - single head with C, D & Eb bodies. Tunable, loud and good intonation. Of the inexpensive whistles, I think these are probably the best. Certainly my D is in tune throughout the range from D to B' when checked against a tuner, though that is not an absolute guarantee, it is a good starting point.

However, for playing out, I prefer Recorder and my wooden flute. I have a couple of "Mollenhauer Dream" Descant Recorders with single low D and C holes which are excellent. Good tone, in tune and quite loud without being shrill.