What's the most accurate measure of mugginess as in uncomfortable, humidity, relative humidity, dew point, etc?

"Muggy" is a subjective term describing how uncomfortable you might be in warm, humid weather. There is no measure to it that isn't completely personal. What I call muggy, you might consider drier than normal. I can call a 70° evening with 55% humidity muggy, and you might be sitting there in a sweater and wondering if the tree next to you is about to spontaneously combust due to how "dry" it is.

One way to determine what's considered humid would be to calculate the 'temperature-humidity index' (THI), also known as the 'discomfort index.'

Studies have shown relatively few people in the summer will be uncomfortable from heat and humidity when THI is 70 or below; about half will be uncomfortable when THI reaches 75; and almost everyone will be uncomfortable when THI reaches 79.

THI = Temperature - (0.55 - 0.55RH) * (Temperature - 58)

THI = 100°F - (.55 - .55 * 0.6) * (100°F - 58°F)

THI = 100°F - (0.22 * 42°F)

THI = 100°F - 9.24°F

THI = 90.76

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The reason the answer is not relative humidity (RH):

Humidity is a dimensionless ratio ... expressed in percent ... of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.Since the latter amount is dependent on temperature ... relative humidity is a function of both moisture content and temperature.

T:  35°F

Td:  35°F RH = 100%

Who among us thinks an RH of 100% feels muggy when the air temperature is 35°F?

Relative humidity.