Have you considered doing an instrumental song? Or is a case of the vocalist thinks he's singing and it's too late to change your mind? 'Albertross' is never a bad bet (Fleetwood Mac's still classed as rock these days, right?)

For songs that have vocals, but the guitar can take over, what about 'Sunshine Of Your Love' by Cream?

Black Sabbath's not a bad suggestion either...'Iron Man' or 'Paranoid' instantly spring to mind, but there's tonnes of other's to choose from. Asch is right, the crowd will be more likely to get into, (and not actually care how good or bad it is) it if they know the song - even if it's shockingly simple...having played in various pubs and clubs, as well at school gigs in my "youth", it doesn't matter how good you are, people ultimately want to hear the songs they know. (Which reminds me, 'Freebird'...consider 'Freebird'!)

Definately look into some SRV or Hendrix songs though, I'm liking the suggestions made already...