AU would be the simplest, and we already can read the format as samples.
W64 is probably the most useful and probably second easiest to implement (both for writing and loading as sample).
OggPCM is easy to write, however loading OggPCM requires a considerable amount of work in order to parse the channel mapping headers correctly and handle the downmixing. Also, it is not used anywhere in practice.
RF64 quite honestly looks like an utter mess of hacks.
WAV with multiple chunks is just way too dirty to consider.
CAF is not considered any more because it is kind of Apple-specific, however it is extensively and clearly specified.

W64: The Cue64/Marker chunk is ambiguously specified with respect to whether it implements chunk-external padding (like all other chunks) or chunk-internal padding. Implementations differ on behaviour in the wild: ScalaAudioFile writes chunk-external padding, Reaper writes chunk-internal padding.
W64: The Cue64/marker chunk uses UTF16 encoding, however all other string metadata inherits the problem of unspecified 8bit enconding from RIFF WAVE. Gloriously inconsistent.

AU export implemented in r9067. Given that none of the considered formats is perfect and without quirks, AU was chosen because of its simplicity.