Lossless codecs like WAV or AIFF are the best quality but rather cumbersome to upload. I'm sure there are subtle differences among the lossy codecs, but usually it doesn't matter. Download compatibility is generally pretty good with MP3's but I don't think iPods accept WMA (could be mistaken) and most generic players don't accept Apple formats like M4A. I always upload my files at 16bit, never tried higher.
Soundcloud will mangle your high end a bit (normally high hats and cymbals), generally youtube and others sound better to my ear.