I'm with Rich04; I go for very balanced ships. Just like with weapons, each of those ship types has inherent advantages/disadvantages. Nimble ships can't be easily hit by Fortresses; Swarms are so small they're worthless against Muscle; and so forth.
A balanced ship has a chance against all other types, I think.
Of course, my true favorite is a swarm of Fortress and Muscle ships. Who says you can't have a fleet of 70 DN's?
LL