Interesting question, I guess my vessel of choice depends on the scale of the battlefield, use of resources and if team-play is involved.
On a full scale map with resources and teams/factions : Probably a self-sufficient logistical ship to keep the fleet supplied and combat ready, allowing them to fully dedicate to a combat role.
Full scale, free-for-all: Some asteroid converted into a gun platform, or more likely a ram.
Arena-scale with teams: A support cruiser, staying behind the tankier ships assessing the battle and taking shot where it'll hurt the most.
Arena-scale free-for-all: I'd choose a Minelayer to be an absolute pain to everyone. If not available, a torpedo frigate.