Which lossless format is best for ripping ?

As usual, there is no answer. It's a matter of personal preference.

Using the uncompressed Wave format is quickest, but will produce bigger files.

Using FLAC will take about a quarter longer, but will reduce the size of files by about a third.

Using Monkey's Audio will take about a third longer than Wave, but will reduce the size of the files by a bit more than FLAC. It all depends on which you value more: time or space.

FLAC offers the best mix of performance and features. The difference in compression size between it and Monkey's Audio is not huge.

Even comparing FLAC's 'medium' setting to Monkey's 'extra high' setting does not produce a difference of more than a few percent. Also, FLAC is very much open source, using an OSI-approved licence, so it should remain free and available without restriction.

And FLAC seems to have been implemented in hardware devices (such as the Squeezebox, the Rio Karma, and the PhatBox) whereas Monkey's Audio has not. This could mean that you decide you don't need lossy audio formats at all, and simply keep all of your music in FLAC format.