Sword of Spirit
Legend
It depends on the setting, though. In the Old Gray Box FR, the super-high-powered monsters/villains/organizations are already there from the jump. The dwarves have been depopulated by generations of genocidal war and are on the brink of extinction. The elves have bailed. Civilization is scattered and hanging by a thread, with powerful enemies pressing in from all directions, including below (in the North, at least, which is the focus of the OGB setting). Basically, the North has been circling the drain for a while now, so it's not like all these threats just pop up in time to challenge your high-level PCs.
I know what you're saying, but I still don't think any edition of the Forgotten Realms ever presented a world populated with enough high level monsters to level a party of characters to 20 using typical Medium-Deadly combat encounters as your major source of XP. Sure, you could fly around trying to depopulate the world of dragons, but running into encounters with a bunch of uber-monsters together, or even running into a bunch of uber-monsters, is a real strain on credibility.