I am considering a new idea, namely an aggressive and arrogant cephaloid race called the Glorph Empire who are partially based after the Martians from War of the worlds as well as several other inspirations.
The have history with Humanity even before humans became a spacefaring race and that and many other incidents cemented the fact they are as belligerent with humans as possible.
they're generally treacherous and known that if given the faintest chance they'd conquer known space space without the faintest hesitation.
Of course with this galactic alliance thing going on they are pretty much kept in check.
They are also known to come up with various doomsday weapons and wondrous measures to promise them victory...except they almost always malfunction and cause more harm to the Glorph than to their enemies.
:lol