Altered Bass-/Slash Chords: e.g. C/D (D3 C4 E4 G4) - seconds and fourths of the chord typically sound better than sixths and sevenths

An altered bass chord (aka "slash chord") is written the same way as an inverted chord, like this: C/D

and looks like this:

The D note is not part of the C chord (C E G), i.e. it's a non-chord note, so C/D just means a C chord played simultaneously with a D note in the bass.

According to Michael Miller in his book The Complete Idiot's Guide to Music Composition, when choosing a "pleasing" non-chord bass note, "seconds and fourths of the chord typically sound better than sixths and sevenths".

This means that in the key of C, "pleasing sounding" bass notes would be D (second) and F (fourth), while A (sixth) and B (seventh) would be less pleasing. Just for clarification, here's the C scale:
C (root) D (second) E (third) F (fourth) G (fifth) A (sixth) B (seventh)

Here's an audio example of the 4 different "non-chord-bass-note slash chords" for the key of C: C/D (D3 4CEG), C/F (F3 4CEG), C/A (A3 4CEG), C/B (B3 4CEG).

Personally, I think they all sound pleasing, but see what you think.