We've got four feeders, and have squirrel-proofed all of them. But you can see them estimating the range for a jump from a tree, or climbing a bush and "having a look".
The most effective way is a tube-type feeder hung on a 20-foot wire from a tree branch, well away from the trunk, and about 6 feet off the ground. They can't climb down it, and they can't hit it on a banzai leap ....
Lannis