The "best" way to get MP3s is different to different people. My idea of the "best" way to get MP3s is to create them myself, by ripping my own CDs. That way I can be assured of a perfect rip, and not have to worry about random scratches and blips which, at high volumes, could damage my speakers. The favorite setup around here tends to be EAC -> LAME.

As for file sharing services, I'm sure someone around here has an idea which are better or more popular.