I have a couple or three D’s, plus a G and an F.
Amar made me buy the F. Glad he did too.
Berti, the Gen.F is high, not low. Really rather nice it is too. I can’t play a Gen High G though, the holes are too close together for my fingers.
If you define “Low” as “below a Gen Bb” I have whistles in A, F, and D. I’m not sure if I consider my A a “Low” whistle though, since I play it with my normal high-whistle grip. I can play my F with either my finger pads or piper’s grip, but I’ve had to learn proper piper’s grip to play my Howard Low D.
Depends on my mood which one I pick up, though since I’m still working on piper’s grip I tend to pick up the Howard the most at the moment. But I think the A is the most versatile - can play in A and D easily, has a good portion of that “low whistle” sound, but still fast and responsive as my high whistles. I can play the F for hours, though - it has an ethereal quality the D doesn’t have. I can’t really pick a favorite, and I’m glad I have all three.
Actually the majority of my whistles are lower… Does that make me “bottom heavy?”… 
My faves are the bass (or baritone) Bb/C Alba set, an Overton set with C#, D and D# bodies, some Kerry lows - (C, D, Eb, E, F, G) an Overton low F, and at the bottom, an Overton bass A. (although I’m also going to be adding some of Daniel B.'s Basswhistles sometime, and would love a bass B to play in E…) Oh, and a Yamaha bass recorder. And a tuba. 
Low D, A,C and intend to get a G soon
F & G are indispensable. But, for reasons that have never been fully explained to me, I like the Low Eb.
Dale
F is great