On Mark Levine's Jazz Theory book, on Chapter Five, Slash Chords, it says:
Although triads can sound good in any inversion, all things being equal triads sounds strongest in second inversion.
Then there are some examples of slash chords, with C as the bass and all the major triads that can be played on top of it (1). All of them are in second inversion and then there is the aforementioned quote.
But why do triads sound strongest in second inversion?
Edit: With slash chords, the author isn't referring just to inversions. The example above shows any major triad that can be played over a C note. For instance, the E/C refers to the Lydian Augmented chord, which can also be written as C maj7 #5.