Time is not metric, ever… and Celsius not Kelvin for temperature. So since you only have 3/5 valid metric measurement types I think you should start over. However, since I want to keep buying milk by the gallon and so much of our groceries come in lbs, I think length (or distance) is the first place to start. Get people accustomed to it when they drive. The apps we all use will speak out loud how far we have to go for certain things. We can start visualizing it without realizing we’re learning.
Mass and time are probably the least likely. Mass is nice because one pound mass weighs one pound force and time just isn't changing. Temperature is a maybe but only for Celsius, not Kelvin, and Celsius isn't even that useful for most things. Volume is the most likely since liquids already have both on their packaging in stores, although distance could also happen since a meter is close to a yard.