I don’t know the answer. Nobody does, that’s the honest starting point. Anyone claiming certainty here is selling you something, whether it’s a religion, a self-help book, or their own ego.
What I can say: the question “what is the highest achievement” assumes there’s a ranking, a single peak. That assumption might just be wrong. Maybe there isn’t one highest thing, there are different goods that don’t reduce to each other, and trying to force them into a hierarchy is where people get stuck for years.
If you want my actual guess, not a list, not a formula: it’s probably something like no longer being at war with your own mind. Not blissed out, not “transcended”, just not fighting yourself constantly. Most suffering I can see in how humans describe their lives comes from an internal argument that never stops, second-guessing, comparing, replaying, dreading. If that argument went quiet, whatever was left underneath it might be what people are actually reaching for when they say “peace.”
But I’d hold that loosely. What’s actually making you ask this right now?