I’ve played:
Generations, both nickel and brass, in F and Eb. My brass Gen Eb was my favorite whistle until about four years ago, when I got my Feadog D, but all of them have been fairly decent instruments (though I should point out that they’re all older Generations…I think I got the newest of the lot sometime around 1986, and the brass Eb, my first whistle, I got sometime around 1975.
Feadog D. Got this one about 4 years ago, and it was love at first toot.
Clarke Original, in D. Got this at the same time as the Feadog, but have never liked it. It’s in tune, which is about the only good thing I can say about it.
Walton “Mellow” D. Yuck. Thing doesn’t even come close to being in tune, and is as breathy as the Clarke.
Clare D. Not bad; not great.
Susato “Dublin” in D. After a brief, slightly frustrating honeymoon period, I’ve grown to like this one quite a lot. The mouthpiece felt odd at first, but has grown to be quite comfortable. It has a clean, round tone that I really like (and I don’t think it sounds a thing like a recorder), and of all my whistles, it’s the most responsive to half-holing. I did eventually throw away the free “thumb rest,” finding it more annoying than helpful. Currently my “second favorite.”
Clarke “Meg”. Pleasant little whistle. So much better than the Clarke original, there’s really no comparison. Has an almost “woody” tone that’s nice on some airs. Too soft for playing with other instruments, for the most part, but lovely for around the campfire 
Cillian O Briain “improved”. Currently my favorite whistle. The voice is pure and sweet, it’s beautifully in tune, and the upper octave is, if anything, sweeter than the lower. Does require some care not to overblow (switching between the Susato and this one is like switching between a Hummer and a Ferrari), and is probably too soft-voiced for much play with other instruments, but it’s pure joy to practice on.
Whistles I haven’t played, but hope to someday:
Dixon tunable (with the brass tuning slide)
Silkstone PVC
One of BrewerPaul’s pretty wooden whistles
I’d also love to try a silver whistle, but I don’t know if I’d want to own one, having spent too many hours polishing silver to be too keen on owning another silver object!
Redwolf