Dont worry, stairway is a pretty simple song once you've heard it countless times.. break it up into segments that just repeat. and just keep practicing.
I think the hardest to sing ive ever come across is stuff by Metallica, like really, how does Hetfield play those metal riffs while singing something with such energy?
That, and of course those songs where male singers go "no-balls mode" e.g. Yellow, anything by Queen. Smells like teen spirit, i just don't know how Kurt does it. His voice is so cool too.