I generally feel the highest value tests are at the lowest level (code with few dependencies) as well as end-to-end integration tests. It gets a little murkier in middle layers since alternate implementations of dependencies (aka mocks) need to be created, or one has to deal with complex setup of dependencies or in some cases re-testing the same logic that has already been tested at a lower level.

I'm not suggesting middle layer testing is not valuable, shouldn't be done, or to never use mocks, etc., it is just much more difficult and more creativity is required to add value in this area. For example, in some situations middle layer testing requires the creation of various kinds of utilities and frameworks in order to find a reasonable benefit vs effort sweet spot. Dogmatic approaches (such as mocking all dependencies in all situations) feel like they are productive at first but can end up being costly as each mock is an alternate implementation of an interface and doesn't necessary capture the behaviour of the concrete implementation.