Comics are fraught with peril as contradictions between writers/editors are rarely resolved and they are often best case considerations. With that said, I will go over the three positive rated classes which are Beta, Alpha and Omega class metahumans. The other three ratings, Gamma, Delta and Epsilon are often overlooked in the comic genre but are occasionally addressed in novels and other kinds of fiction. See the Wild Cards series and their terms Aces and Jokers.
How you gauge a metahuman (in most modern comic universes)
A quick rule of thumb is a metahuman that can only affect one person at a time, only some of the time or with some other limitation is a Gamma. They are likely unskilled in the use of their powers or their powers are simply too weak to be a deterrent to the current level of technology. Gammas can be brought down in a variety of ways and overall are not considered a threat, except to the unaware.
If a metahuman can affect one person effectively or larger number of things or people intermittently, then they are a Beta. Betas may be more effective, but can usually be counted with a minimum of force. Most Betas who remain untrained will simply use their powers for personal gain because they are aware of their limitations in comparison to any Alphas who may exist. If no Alpha’s exist, a Beta could be a threat if they orchestrated the circumstances of the power use well.
If a metahuman can use his powers all of the time, in a variety of ways, affecting more than one person at a time, for an extended time, the he or she is likely an Alpha. If a metahuman is able to do extraordinary things completely unable to be replicated by machines or current technology, they are an Alpha. If a metahuman can create things that were never seen before they existed or conceive of things in ways, previously unknown, and do that regularly they are likely an Alpha. In a world without other Alphas, an Alpha is nearly a godlike being depending on how their powers manifest, and how they use them. In our world, Alphas would be conscripted by governments or killed by assassination details, if it was even possible to kill the Alpha. Alphas usually have one or two weaknesses they keep under wraps.
If a metahuman can use their powers and effect dozens, hundreds, thousands or in the exercise of your powers affect the lives of millions, they are likely an Omega. If a metahuman's powers could conceivably destroy all life on Earth and there is little or nothing that can stop them they are likely an Omega. In the MU, this is the most feared class of metahuman. The DCU is only a little less paranoid about Omega-class beings.
Example: The X-man codenamed Cyclops (Scott Summers) is a Silver Age, Beta-class hero who has evolved into a Modern age, Alpha-class hero. His single power, his optic blast has grown into a diverse energy power capable of being described as: Energy Projection, increased damage, extreme range, capable of deflection (using his optic blast), indirect attack (bouncing his optic blast off of other substances), precision attack (able to control its strength, duration and amplitude from flipping a coin to destroying a mountain top).
Cyclops supplements his physical vulnerability using an armored uniform, hand to hand combat training, and regular intensive combat simulations to hone his skill with his power. As a member of a team engaged in the same intense training and supportive combat systems, their group synergy enables them to be even more effective than their individual power levels would suggest.
There are the occasional non-powered or technologically assisted heroes who can move out of the ranks of the Gamma or Beta class to be considered Alpha-class material due to their preparation, quality of technology, or are simply just so bad-ass they can hold their own against superior metahumans. Bronze Tiger, Black Panther, Captain America, Batman, Green Arrow, Hawkeye, Lady Shiva, the new Batgirl are all unpowered humans whose prowess or skills allow them to interact at a higher level than would initially seem possible given their unpowered state. They will never, however, be considered Omega-class beings without outside intervention such as the introduction of the Uni-power.
Challenges in defining power levels
The comic industry is part myth-making, part storytelling, and it is difficult to gauge the power levels of the beings talked about in those stories efficiently when they were created at different times, by different writers and often managed over many decades and many character iterations, reboots and continuity transformations.
Numerical designations (i.e. "a 5 on the power scale") are the artifacts of writers who are unfamiliar with previous work done in the comics they are writing. Much to the chagrin of many fans, some writers do not read any of the previous issues of a comic before they come onboard. So they create a "rating system" to assign a threat level using numbers. Same idea, different system, and the results vary by writer.
Comic fans needing order and structure, attempt to classify the heroes and villains even when the companies who create them would rather not, based on their reluctance to release documentation in relationship to the characters in their product lines. This dichotomy created an eventual need to create classification attempts such as the Marvel Universe Handbook, or the Handbook of the DC Universe. While those publications sold well, they did little to reduce the arguments of fanboys about which metahumans were the most powerful of them all...