As a player and DM through four editions now, I have observed the effect commonly referred to as “the sweet spot.” This is the point at which the game functions as intended, is fun for players and DMs alike, the PCs can handle most fights, and the monsters can be threatening without being overpowering. While the sweet spot in 2e was a strange one due to the difference in classes’ experience charts and earning, and 3e and its iterations had multiple sweet spots, 4e to date has had the longest, most sustainable sweet spot that I have experienced. It starts around level 3, and lasts until about level 23.
However, within this sweet spot, there stand four levels which I feel are the best time to be an adventurer in D&D. While three of them stand firmly in the paragon tier, being levels 12, 16 and 20, one stands in the heroic tier of play. Level 6 is the goal to shoot for in lower heroic, and it is where the PCs truly begin to feel their power. It’s when character choices in build, theme, and class truly begin to differentiate themselves. It is when heroic characters can get their first taste of the paragon tier challenges ahead, and the best time to start defining a campaign’s themes. It is the golden level of heroic adventure.