Which one of the lossy file formats like Ogg, AAC, Mp3, Wma has the best quality/size ratio? I've seen that AACs are smaller than MP3 at the same bitrates. Do they sound worse than MP3?
You may find a lot of subjective answers here, and some of the quality seems to be dependent on what you play the files on - for example AAC and low quality mp3s will be fine on your iPod through headphones, but pop them on a decent system with good speakers and they will sound crap (Skeptics question here)
Each of the lossy formats has its own way of deciding what to lose (the wonderfully named psychoacoustics) and you may prefer one over another.
I don't like the way AAC, Mp3 or WMA compression sounds on low or medium bitrates, but on high I can't tell the difference between them. Ogg I actually like slightly less at low bitrates than the others, but morally I like it :-)
So I use mp3 or ogg in all my audio players, at high bitrate/quality settings (or flac in the devices that support it)
A good place to look for this type of info is Hydrogenaudio. But... because this is so subjective (quality) it ultimately comes down to doing some listening tests yourself.
As a purely pragmatic thing, I'd use .mp3 for lossy compression as it's established itself as the gold standard for better or worse.
I personally think that WMA or Ogg sound better at lower bitrates than MP3. At higher bitrates I don't notice as much of a difference.
For my MP3 player I tend to encode everything at the smallest file size I can without constantly hearing the compression artifacts.
I currently am using Ogg at compression level 0 which is roughly equivilant to 64 Kbps. The only thing I don't like about the format is that Windows Media Player does not like it, so I must add my music to my MP3 player manually instead of being able to sync using WMP.