For me that spot tends to be pretty low around 6th or so. This is the place where the game is action oriented but still human. Regular stuff like say wolves or bandits can still challenge the party and its not "monster of the week" or "How come we never saw those before?" but OMG Orcs!
Its what Ryan Dancy calls the 1st (and maybe early second quartile)
Levels 1-5: Gritty fantasy
Levels 6-10: Heroic fantasy
Levels 11-15: Wuxia
Levels 16-20: Superheroes
Gritty to Heroic
I guess this comes from a steady diet of things like GURPS and Runequest where combat can be deadly. Of course I also want my fantasy to have verisimilitude, the notion that "well that could happen"
Now this sweet spot idea is nothing new,.
The older editions had an implied one in the form of an end game that capped hit points at levels 9-11 and abilities that lagged back a bit as well. Not much point in being past14th as a thief. 95% at everything is pretty awesome and it doesn't get better than that.
Spellcasters of course broke this mold with higher level spells but on the whole D&D was calibrated to "Pretty Heroic" as vs the Superheros we see in the last few articles I've written.
Now of course there are ways to alter the game to lengthen the sweet spot, E6 the game within D&D is decently well known (thread one, and thread two and great for those who like lower level play (L1-L6 or so) . There is also an E8 Variant and even an E10 has been bandied about
Another technique is to limit the spells list to remove abusive spells and spells that really effect verisimilitude.
This can be a bit of work but if you like higher level play (just not some of the spells) it can be worth it. Which ones to disallow is up to you but the usual ones are the ones that require magic to overcome, anything that creates stuff Ex Nilho (no more wall of Iron w/o a time limit) anything permanent (including Continual Flame) Teleports and Raises.
I've also seen people limiting spell casting classes to once every two levels, so that a 20th level type can have at most 10 caster levels. This can be cool as it encourages some the more mediocre prestige classes with limited caster levels that we seldom see such as Spellsword
All of these work to one degree or another .They key is to pick which you like, your players like and run with it.