It depends on what I'm doing.

Baking: metric - grams on a food scale help me replicate recipes perfectly.

Cooking meat: standard - 140°F steak, 165 chicken

3D printing: metric always

Woodworking/household: standard - inches are just the standard for all the things. Wood, curtains, hardware, etc

Mechanical: ??/?? Depends on where it's made! I hate having to switch wrenches or sockets due to the wrong standard.