Marza for TEC.
"I beamed them down on the planet. I stayed behind. Last man on the ship, thats what you are supposed to do as a captain, isn't it?" - "Where is your crew?" - "On the third planet..." - "There is no third planet." - "Don't you think i don't know that? There was but not anymore... if you had seen it you'd know the whole thing is a weapon, it must be... its miles long, it destroys planets, chops it into rubble..."
Devastator for Vasari.
"Tactical analysis." - "52 disruptor banks, 27 photon torpedo bays, primary and secondary shields." - "Its a predator."
Halycon for Advent.
"Admiral, we have enemy ships in sector 3-7." - "Its a trap." - "Fighters are coming in." - "There is too many of them." - "Draw the fire away from the cruisers."
I have to admit colony-capitals are always an alternative especially on large maps. But the colony aspect aside thats my pick.
The Marza is simply lethal if it manages to slip through a planets defensive perimeter and its well geared for doing so. The Devastator is simply the most powerful in ship-to-ship combat around.
But my absolute favourite has to be the Halycon.
Its capable of amplifying all that nice beam damage from your Illuminators and can be geared to either destroy frigates and provide cover with fighters or to kill capital ships and heavy structures with bombers. Usually early on you want to kill frigates and later assasinate capital ships.
Also it can deal with fighters on its own. And of course its the meanest carrier in the entire game.