My Lamy 2000 fountain pen is my primary writing tool, but there are situations where a fountain pen is impractical, such as writing a quick list when outdoors or scribbling an idea onto a notecard. I like to carry a backup pen for this. For me, that's the Fisher Space Pen. It's simple, small, yet by posting the cap on the barrel when writing it extends into a full length pen. It's famous for being able to write upside down, in space, on burning metal, underwater, and probably a combination of all four; but none of that is interesting to me. It feel's good, it's convenient, and it works without you noticing it - surely a sign of superlative design.
I did dabble with the Lamy Pico ballpoint pen, which is similarly compact and well-designed. However, it's a little less discrete than the Space Pen and I'm concerned that compatible refills are harder to come by.