The ones I like, and keep buying, are the Oak/Acorn line.
Oaks have a nickle plated tube, and the Acron has a brass tube. The head seems to be the same.
I prefer a brass tube as it’s not as slippery, but most folks like the look of a shiny silver tube
These are sold at the local Big Box Music Store, ours is next to a Trader Joes that we frequent.
I like these enough to give to young people and relatives who show any interest.
And your opinion is…?
The last time I played an Oak I had a bad nose bleed. Ominous? Telling? Certainly overpriced…
I bought a dozen of these recently to give to aspiring whistlers:
Surprisingly, they’re in tune and fun to play. (Too light for my tastes, really.) Also, there’s a tweak available that make them comparable to a fog horn.
Impress your colleagues with your thriftiness and manliness with this little schwartz colored gem.
Waltons little black is a nice whistle out of the box. I give them away to coworkers all the time. I do stick a bit of putty in the head cavity though. Plenty of volume and not shrill at the top.
ecohawk
I find the plastic whistle made by Woodnote to be the easiest under $10 whistle to play.
Waltons Irish Whistles. I like 'em so much that I’ve recently stopped playing the ones I make and am using just the two Waltons (C & D). That way if something doesn’t sound right it can’t be blamed on playing 7 different whistles. Although at this stage the player should be looked at 1st before even thinking of accusing those poor little pieces of holey pipe. 
My favourite high D, a Feadog MK1, and my favourite C, a Generation, were each bought for well under $10… but that was back around 1980!
The most I ever spent for a whistle was around $500 and it was horrible. I sent it right back to the maker.
Oh, I like $3.99!
I’ll have to try those. Thanks.
me lipth
Feadog Mark I (like PCP above) bought for around $5 in a gift shop many years ago. It’s one of my favorite whistles - regardless of price.
I’ve got a lot of whistles 20+ years old or older. Many of those were well under $10 when purchased. They all have their own charms. One old Oak is nice, as are most of the Gens and older Feadogs. No tweaks on any of them.
My old Susato D and C were maybe $11-12 and seemed like a real bargain at the time I purchased them.
I recently bought a Susato Oriole for not much more than $10 - promotional pricing, I guess. It is a peach of a whistle even at just under $20. But that’s pushing the dollar limit.
Feadoggie
No tweaks, Feadoggie?!?! 
I had to do all the tweaks just to keep from becoming homeless. Maybe in a few more years (or a decade) I won’t have to.
No tweaks!
On another hand, I have owned only one Waltons/Soodlums whistle that didn’t need to be tweaked in some way. Luck of the draw?
Feadoggie
My favorite is my old Feadog. It has a split in the middle of the ramp held together with tape and super glue. The ramp has been filed with a strip of emery board that took off a little bit of the blade and slightly rounded it. I have other Feadogs that are not modified that play well. On my desk in front of me there are a Meg, an Alba Q1, a Glen Schultz water weasel, a Burke brass session D, 2 Feadogs, and a Clare. When I feel like a tune I reach for the old Feadog.
Megs
My green Clarke “Woodstock” I found in a toy store for $4.95. And I found a plastic Dixie whistle at a garage sale for $.50!
Both sound better than I imagined they would!
Generation red top brass Bflat. Bought at Chester Folk Festival about 13 years ago for £2. Clear and mellow.
Generation red top brass Bflat. Bought at Chester Folk Festival about 13 years ago for £2…
Nice…