It is said that lower fractions = greater consonance. 3/2, The Perfect Fifth, being the smallest fraction, has the highest consonance. However it is also said that 12TET has nice fifths, because they are close to Just fifths. In terms of cents difference, this is true. Just is 701.95 cents, while 12TET is 700. In terms of pitch, many people would not be able to hear a difference. Yet as a fraction, one is 3/2, while the other, unrounded, is 1498307077/1000000000. In terms of consonance, or how nice they sound, this would suggest a huge difference. So what is the better guide to consonance here, the small fraction, or the cents difference to Just?
The guide to consonance here is the amount of difference between the tempered interval and the just interval. How complex the fractional difference is, is not audible; how close the interval is to a small-integer ratio is audible.
Of course, perception of consonance is also influenced by other factors, including the source of the tones, cultural and personal tastes, etc.