Here're your ratings:
An unrated virus, generally, is a very minor, easily disposable one. Otherwise, it's immensely dangerous, but that's very unlikely.
Anything lettered or titled is categorized as a "special" virus, which means it's likely huge and in charge of other smaller ones. They go as so:
C - Standard class virus.
B - High risk standard virus, approach with extreme caution
A - Alpha Virus, breeder of other viruses. Deal with extreme prejudice.
Categories One through Five - These are viruses that transcend the norm and are capable of infiltrating the data of human beings (not Moderators, who are made immune by their Initialisers.) They are the "colossi" of which the ancient Moderator myths talk, sometimes being the size of towers, othertimes cities. They are controlled directly by members of The Morbi as mobile war machines used in the sacking of cities. Their immense size means it's rare there's ever one at a time, though.
Category Omega - Only one has ever been recorded, and it escaped into the oceans before it was defeated. Category Omegas are not one virus, but a collection of hundreds of thousands of viruses moving as once.
They devour data faster than anything else on record, move quicker, hit harder and are essentially "the end" for which they are named.
A Category Omega is so deadly that it is advised that no lone Moderator Sector combats it. There is a continuity plan, named "Project Babylon", intended to be used instead, in which every Moderator sector will be temporarily transported to the battlefield in order to combat the threat.