Best investment to defend your planets?

Whit a limited amount of resources what is the best spaceship to research and build if our intentions are only for protection and we are not planning any attack?

Looking at the numbers of every ship and this information is not obvious for me. Off course the best option is a mix of several ships, but I'm talking of a small civilization with limited resources and probabilities of being attacked in the near future.

Any advice is welcome, thank you!