The Omega Cannon is believed to be the Alimbic's "Ultimate Power". It fires a powerful but slow-moving ball of radioactive energy that has a timed detonation, traveling a significant distance before exploding, unless it comes into contact with an object, in which case it will detonate immediately. When Samus Aran fought Gorea at the Oubliette, she used this weapon to defeat the beast's final form, which was impervious to all other weaponry. Heavy gamma radiation emanates from the Omega Cannon, which suggests nuclear firepower.
It has an exceptionally hig damage, as it is a slow moving attack, it is best to use it as a close ranged weapon, but since it is so powerful, it can damage the user too
The omega cannon is perhaps the most powerful weapon in all the metroid series
here is a vid I found on youtube, excuse the low quality: (The cannon´s upgrade appear at the 5 minute mark and so on)