The most cost effective early ship for deterring AI attacks is the starfighter: 50 cost, 1 maintenance, 3 attack. You can hold the computer off indefinitely with these, just make sure you continue to produce them so that your military power is higher than any comp's.
Later, the most cost effective is the anti-matter missile: 300 cost, 1 maintenance, 100 attack. If the AI is putting too much pressure on you for starfighters to continue to work then these puppies will take you to the end game, they cause your military score to skyrocket.
Caveat: you musn't let yourself get attacked using these for defenders, as they don't actually defend for crap. The a-missiles are good for offense if you are attacked, but each one you shoot lowers your military power (because they are one-shot items) and you risk getting locked into an eternal war that way.
Scouts absolutely rock early on, I have my first colony ship's planet build 4 in a row, those combined with my Hero can uncover those precious resources so that I can grab the majority before the AI does (they are also good for quickly getting to nearby clusters to discover whether they are empty or occupied). But they have no military significance, they are...scouts.