If you can find one, a Kahles rimfire 2-7 is hard to beat. This one on a Cooper Classic .22LR.
Ultimate, I don't know. But IMO, it would be hard to find a better squirrel hunting rig, rifle and scope.
I know I like it.
DF
It would be better if it was chambered in 17HM2.
For a while, at least, 17HM2 ammo was hard to find. I have an Annie 1717 in 17HMR that has on at least one occasion shot 1/4 MOA with CCI TNT ammo. It'll go 1/2 MOA with most ammo, likes the TNT best of all.
How does the 17HM2 do on squirrels? I would think it would tear them up more than a .22LR.