A good quality micrometer. Some will go for the classic Starett, others will get a modern digital Mitutoyo. Doesn't matter if it's a lathe or mill guy, CNC or manual. Any decent machinist will have their mic.
Not all of my metrology equipment makes it back in the tool chest every time I'm done, but my Brown & Sharpe digit mics? My Starrett No. 220? My Federal indicating micrometer? Those never see a second out of their case that they don't need to see.