The only way to find this out, is testing this for yourself. All you have to do, is making an A/B test with files encoded at your favorite bitrate, and as lossless as well. even better would be a blind test, were another person chooses the files for you, so that you don't know if you're hearing the lossy or the lossless file.
Cause as Esidarap said, it's totally up to you, if you can actually hear the difference.
Its actually kbps (kilobits p/second). Also it kinda depends what kinda phones u are using, if its amped or not, blah blah blah. Unfortunately the only person who can really decide whats the best bitrate is YOU my friend. Try encoding a song u are familiar with in multiple bitrates, listen to em and see if you can tell a difference. I have an unamped ipod and I use 128 aac vbr on most (not all) of my stuff. I can still tell a difference but to me it sounds good enough at that bitrate.
Oh give me a break. You need to find out what you like, not what others prefer. Ultimately, it is you who'll be doing the listening. I prefer lossless/wav or high-bitrate AAC/MP3. Rip the same track into different formats and see if you can tell the difference.
Speed Metal or really freaked out Freejazz will propably contain more "information items per second" in itselfes than i.e. some laid back Chillout piece. Thus 192 kbit should be enough for Electronica and related, but maybe not for more complicated styles like the above mentioned.
I personally regard the 192 kbit MP3 or 128 kbit M4A quality as sufficient.
I would also say preferably 192 kbps mp3's would be the minimum. But it must be said you only really know what u are missing when u listen to the original. The higher the bitrate, obviously the better.
To this I would add, especially at lower bitrates 128-192kbps range, I have experienced that ogg format does a better job of keeping high freuquency components compared to the mp3. Things like cymbals, high hat etc are noticably clearer and not washy-shouding with the ogg format.