As the site is currently getting hugged I will say that Nausicaa didn't quite leave as much an impression on me as Grave of the Fireflies, but Miyazaki's environment design is always spot-on (the entirety of Farewell My Beloved Lupin, for example, improves on every rewatch as you notice more details).
Grave of the Fireflies is the most emotionally disturbing movie I have ever watched (including non animes). It is a masterpiece but I wouldn't really recommend it to anyone.
It was definitely not a barrel of monkeys, but I think it is an important expression of the costs of war on the innocent, and the effects of societal inequality in general. It's powerful in a way that statistics cannot deliver. You'd also be hard pressed to find a better story of unconditional love, duty, sacrifice, and maturity.
The two children in Grave of the Fireflies are fictional, but they're standing in for a tremendous number of real Japanese civilians killed by these events - in some cases literally starved to death. The Americans really did drop incendiary devices on those cities to burn homes to the ground, in our real world. You can certainly make an argument that it was somehow "justified" but it's a terrible thing to have done even if you're quite sure that it wasn't a war crime (which I am not).
Oldboy is a well-made film, but absolute pulp. It's disturbing in a Silence of the Lambs, manufactured drama sort of way. Your adult amygdala should probably not be disturbed by it all that much.
