For example, with Gohan, I would say:
Has brought important help during the Tournament of Power, therefore being partly responsible for saving Universe 7 and ultimately eight different universes.
Has defeated Cell, a main villain of an entire arc that would have ended human life on Earth, when nobody else could have been powerful enough to save the day.
Has pretty much achieved his dream life, having a big house, a loving family with a wife and a kid, the kind of job he always wanted and being recognized as being excellent at it apparently.
Of course, the order of things is always debatable and depends on your perception of what makes an achievement more valuable than others.
People could consider that those very same achievements from Gohan should be in a different order from most valuable to third best.
But it doesn't matter, it's still interesting to know the best achievements you personally attribute to various characters so far, regardless of the criterias you take into account.