Speed is inversely proportional to importance. The transiting Moon aspects everything in your chart every month and describes an afternoon. Transiting Pluto touches a natal point perhaps twice in a lifetime and describes a decade. Rank by the slower planet, always.
Then rank by target. Transits to the Sun, Moon, Ascendant and Midheaven are felt; transits to Neptune are usually not, because your natal Neptune is shared by everyone your age. Angles and luminaries first.
Finally, applying beats separating. A transit still tightening towards exact is arriving; one moving away has already delivered its material and you are reading the aftermath. Most anxiety about transits comes from reading a separating aspect as a warning.