Defiant, all the way.
If you can, spring for the C-store variant. I believe they call the captain-level one the "tactical escort refit." Then you get more console space, and the phaser quad cannons.
If you can spring for a bit more
, get the LC-level +1 Saber (Gladius or somesuch) and steal its Impulse Capacitance Cell universal console. Then get the Commander-level Excelsior (Advanced Heavy cruiser?) and steal it's Transwarp Computer universal console. Put both of those consoles on your C-store Defiant Refit, and you'll love it.