I like "Hole in the Sky" most among Sabbath tunes, but I'll always have a special place in my blackened conservative heart for "Electric Funeral", since that was the first entire song I learned on the guitar. (Obligatory, once I got my first crybaby wah pedal.)

Among Zep, I like "Achilles' Last Stand" and only really their stuff from Physical Graffiti and later. Their earlier stuff is either played to death on classic rock radio or is so derivative of blues that I find it best to just listen to the source material directly.