Those are all different guns in terms of weight and shape, so I think your first order of business is to find out which one seems best to you. Barrel length options are also something to consider.
Here are some things to think about:
1) The 617 is a heavy pistol. Not too heavy to carry, but significantly heavier than the Ruger. The extra weight (along with the good accuracy) is actually a plus in a hunting gun. Alternatively, you may be able to find a used model 17 .22 that does not have the barrel underlug that adds the weight. This makes for pleasant field carry if you're just looking for something to holster and carry for occasional plinking and opportunistic small game shooting.
2) The 617 is tapped for a Weaver scope mount. This is a significant enhancement that you may want to use at some point. Hunting squirrels in the dim woods with iron sights is a very haphazard thing to do. I personally cannot use iron sights effectively in a lot of real-life woods hunting situations because of dim light of early morning in the woods. This is prime hunting time for squirrels and rabbits.
3) I didn't like the Ruger because I never could get a consistent grip on it. This played havoc on my field accuracy. The double action design fits me much better. Other people are the opposite and prefer SA's. Others can handle both equally well. I am fine with full size SA's but the small frame of the Single Six did not suit my big hands. The Ruger had good inherent accuracy, but I could not make good use of it in the field.
4) Taurus is a copy of the S&W, but with some configuration differences and features. Quality, by all accounts, has varied considerably over the years. I don't know what it's like now from personal experience because this spottiness in their history has kept me away from that brand, even though they do have some very good concepts for guns.
5) Barrel length really matters. The Single Six with 6" barrel was a nice size in terms of weight and balance. The 617 in 6" barrel is very good. The 617 in 8 3/8" barrel is too heavy and long for me. I consider 6" barrels to be optimum for revolver barrels. Other guys think otherwise. Many prefer the longest barrels for use with iron sights because of the better eye relief for the sights.
6) If you don't want a scope, then get a gun with a fiber optic front sight and white-line or similarly visible rear sight. It makes a huge difference in real-world hunting situations. I plan to get a fiber optic sight for my 617.
7) If squirrel hunting is your main fare, then consider a T/C Contender. These are great squirrel guns. They need a scope to make them work optimally. 2x is the magnification I use for my handgun hunting of small game. I only use higher magnifications for such things as prairie dog shooting at long distances with centerfire handguns. I also use higher magification on the rare occasions I'm doing load development or accuracy testing.
Ammo is important. I like Winchester Power Points a lot.
9) Single-action revolvers are more of a nuisance to reload than double action revolvers because you have to unload the empties one at a time. This can be a bother for plinking. It's not a huge bother, but it's something to consider.
10) If you don't like the grips on the Single Six, then your after market options are very limited. I couln't find any for mine, so I ended up selling the gun. If you don't like the 617 grips, there are many options.
11) If I had it to do all over again, I'd pick the 617 again. I've had it for several years and it has been very good. It is not without flaws. For example, the rear sight from the factory could not be adjusted to hit with a center hold. I had to have the rear sight blade replaced. The hammer strike is too light to reliably fire some cartridges. It's gotten to the point lately that I need to adjust it to get it to fire reliably again. In the six-shot configuration, it can be a bother to eject the spent cartridges. Not all of them will eject reliably and I need to manually pry the spent cartidges out with my finger nails. Pushing the ejection plunger sharply to eject the spent cases helps, but does not entirely solve the problem.
