sweet spot

Short definition from Wiktionary

Noun

1. 1
   Any place that is optimal for obtaining a certain desirable effect or result.
2. 2
   Any set of conditions that is optimal for obtaining a certain desirable effect or result.
3. 3
   The clitoris, prostate gland, or other internal center of sexual pleasure.
4. 4
   Any of several places on a bat, racquet, etc., with which to hit a ball, resulting in the latter rebounding with some optimal desirable effect, such as minimum vibration or maximum velocity.
5. 5
   The spot of a lens in where image clarity is perfectly clear and sharp.