This can be correctly only applied to things and events. It roughly means "producing impression", "producing emotion". It can be applied to night, tour, glance, taste, eyes, amusement etc.
This is very high-style. A Church-Slavonic borrowing. Roughly translates as "exceptional", "blazing", "outstanding". It can be applied only to actors, poets or heroes and their deeds. This implyes that those people had perished or had not been seen for a long time. This implyes their impact is still substantial yet the people most likely are not currently widely known, they just sparked in the past. They are more part of eternity rather than current events and discourse, yet memory of them is somehow kept by the followers or monuments. Applicable to people, deeds, sacrifice, heroism, victory, popularity, recognition and other achievements.
You cannot apply it to people of the past who are very well known, like presidents or national leaders of the recent past. It is applicable only to relatively half-forgotten yet exceptional people. It has a taste of sore and pittyness, maybe melancholy and a lot of internal strength.
To produce super-duper high-style you can combine several Church-Slavonic borrowings: незабвенная клятва, незабвенное знамя, незабвенное пламя etc.
In sum, незабываемый means unforgettable, impressing, while незабвенный is... infallable to oblivion (забвение means oblivion and similarly high-style).