Here are some basic guidelines to follow:
1. It can be anything, so long as you can justify it as a "superweapon". Heck, it could be a <<+!!super Sephiroth cosplay army generator +3!!+>> for all I care.
2. For the basis of this discussion, the rule of "always build at least three superweapons" can be ignored at your leisure.
3. Try to be honest with yourself. It's hard to describe something you made up to look "cool".
Let me start (I think everyone knows what I would put here):
Portable self-sizing giant orbital cannon array.
It would consist of 9 self-sustaining superlasers (each with the ability to engulf or shoot through a planetary body the size of Jupiter) that can either orbit a system or move about at will; an independent satellite system of sorts. These lasers would be of a continuous firing sort, not one shot beams, which can be redirected and resized at will. They can also be focus fired for increased, incredible damage.
These superlasers will be protected each by individual self-sustaining mini point defense systems that will slice through anything that approaches with (hopefully) perfect accuracy.
It's final ace in the hole will be that the superlasers and retinue can freely resize into any shape or form I desire and is capable of firing its arsenal through optional portals in the gun barrels that exit wherever I wish so as to maintain perfect stealth. (And also to trick assailants into thinking I have t3h l33t psychic powers of giant laser beam)
In short, the only weakness that I can think of for this system would be me. That and a total galactic meltdown.
