I've just sent off a request to client services that they consider FLAC as well, latest manual page (attached) I got sent doesn't include it. Gave them same feedback last year when I took delivery.
Choices are actually pretty limited if you want highest quality AND to take advantage of full browsing and album artwork.
WAV and AIFF (aka Apple lossless) are lossless, but don't contain any meta data (album covers, titles, albums, artists etc). WAVS work and play really well BUT the system only recognises them as discrete 'songs' and treats the filename as the song title which makes navigating and selecting by album all but impossible.
MP3 contains the metadata but is compressed and lossy (which won't do justice to the Meridien system which as we know isn't very forgiving of poor quality source material)
WMA/ASF I have very limited experience of and I'm not sure if they do contain metadata.
The other formats are I think native to linux or unix systems so about as useful as the radio.
All my current collection (15,000 odd tracks) are in FLAC lossless format for use on my Sonos setup at home. At the moment I convert FLACs into WAVs for the albums I want on shuffle and just play off a USB stick.
FLAC would be perfect compromise (are you listening Woking?), open source, high quality lossless, smaller file sizes than WAVs AND contain metadata necessary to properly navigate IRIS and display artwork.
Short answer for the o/p :
if your priority is browse and play tracks by album or artist you can only use MP3 so rip it as high a rate as possible with as little compression as possible but you probably won't do justice to the Meridien's quality.
if your priority is sound quality then use WAV or AIFF, but you'll have to do some clever naming of your filenames to get anything vaguely browsable.