Low D is always good.
Bb is still my favorite key. Someday I’ll have to try a B Nat thought.
I prefer Eb over D simply because of the extra kick it brings.
I practice outside exclusively - not welcome in my own home - and I’ve found that the natural air pressure makes a big difference in how each whistle sounds. It’s a nuisance when the wind is gusting - I must look like a weathercock. I carry a sporano D, C and B natural. Sometimes I’m in the mood for mostly D, but the weather won’t take it, even if I adjust the fipples (these are home-made PVC jobs). I try them inside, and they’re fine. If it’s a fine, hot day, all the whistles sound good. If it’s dull and rainy, they still sound good as long as it’s not too windy.
So my answer is, it depends on the weather!
I like whistles in the range from C# to Bb simply because they fit my hands best and because I haven’t become a fan of low whistles. I have big hands with a right index finger crooked from an old break, and my fingers are a little too close together on the high D. For months now I almost always play the C# because I really like the sound of it. No other reason.