Back in the day, when we were teenagers and played D&D almost every day, my favourite games to play were those around levels 4-7. Reasonably beefy characters who had gathered a few magic items and could go up against a range of foes including dragons, hydras, giants, etc., and yet could be (and often were) brought to ruin by level-draining undead or a failed save versus poison.
Nowadays, I find the brutal, hardscrabble, survival horror of 1st-level adventures, in which a single spear-thrust can kill and resource management is key, to be strangely satisfying, and I have renewed respect for the triumphant achievement of a character gaining 2nd level. That said, I'm still pleased when the characters gain a couple of levels of "cushioning" and their tactical options begin to expand. At 5th level, magic-users can start casting spells like Dispel Magic, Fireball, Fly, or Water Breathing, all of which are (literally) game-changing. When I have ideas for adventures, they tend to be suitable for 3rd-5th level characters, say around the 10,000 XP mark.
What's your "sweet spot" for adventuring in terms of levels?