If you want to maintain the best possible compatibility (on mobile devices) without losing quality, you're probably best just sticking with WMA, honestly. You could do FLAC but device compatibility with FLAC is spotty compared to WMA, and FLAC takes up a lot more space. If you want to maximize compatibility with both computers and mobile devices and aren't concerned about losing some more quality, a high bitrate MP3 conversion is probably best.
On the other hand, if your only concern is playing these files on PCs (not portable devices) then vorbis and FLAC are both fine - though as Obed pointed out, going to Vorbis will lose more quality (just like going to MP3) and as I pointed out, FLAC takes up a ton of space.
There's no real "best" answer here.
Just out of curiosity, how did you end up with so many WMAs?