Nikkor 50mm f/1.4D
This lens finds itself on my camera as much as any other lens. Cheaper than most other fast primes, I like it a bit better than the f/1.8 version because it has a bit more subject isolation (duh... 1.4 vs 1.8). It has a dreamy quality wide open which takes a bit away from sharpness, but stopping down a bit fixes it if that’s what you’re worried about.
I find myself enjoying the unique feel to the lens at f/1.4, as well as it’s any-purpose focal length. Small, lightweight, and fast. Perfect for stuffing in your pocket to complement a wider/longer focal length.
I’ve found myself using primes more often than my zooms... they’re just that much better/more useful for most purposes I find myself in.
An extremely versatile lens, I find myself thinking occasionally “...should I have just popped on the 50mm instead?”