Death Star.
That said, it had an incredibly vulnerable flaw where some punk kid could bring it down with a lucky blaster shot to its innards.
The Enterprise was VERY vulnerable to phasers when it had its shields down.
The "Planet Killer" in the Doomsday Machine, had a great deal of power until a Constitutional class starship blew itself up inside.
I guess it's a Superman vs. Batman thing.
Debatable, but cool. For us geeks.