not sure if any of them are strong enough to drop a squirrel unless ur close enough or hit the right spot but daisy makes a good one as well as airsoft those r the 2 best brands i know of and im not sure but they may even have some that are more powerfull then others
Beeman is a good brand. Walmart has one with a .22 & .177 interchangeable barrels and a scope. It's about $100. The .177 pellets go at about 1000 fps. I've taken out several squirrels at over 40 yards.
Just make sure you have a clean enough shot that you kill the squirrel quickly and humanely. If you ever have to finish an animal that's thrashing around and crying in pain because of your poor shooting you will understand why this is important.