I was talking more about HW2. If it was HW1 I would say Heavy Cruiser, or if it was HWC I would say Dreadnought. And as Ante said, the way the tech is set up, and intel, velocity, everything, it wouldn't play out. A capital ship wouldn't even be in visual range to fire.
Frigates are used to scout the area, as in ME terms, a battle takes place at ranges where light and LoS based targeting are ineffective. Every shot has to go where its supposed to, due to a travelling mass driver slug potentially impacting a planet EVENTUALLY. It even says that within a certain range (some number of kilometers) dreadnoughts don't even fire their main cannons, in fear of the explosion destroying themselves.