Literally hundreds, and most of them I can legit sing/rap as well.
Comes with being a musician who plays in cover bands as well, and lots of karaoke. Though, I'm usually just a backup/harmonizing vocalist
but when playing covers, you learn to take a lot of cues from the vocals, and in turn, learn the lyrics quite well.
Hardest song? I can rap the entire Deltron 3030 album cover to cover.