Willie Johnson's classic is a one chord tune with a slide guitar riff that closely mimics the vocal line.
Try and play it up-tempo if you can which means you'll have to be very accurate with your slide particularly. The trick to this one is in following the phrasing very closely to get the melody tight around that constant downbeat on the thumb.
An 8 bar slow blues from Elmore James, probably the most influential slide player of the 1950's. The licks in this intro represent the foundation of his playing and countless players after him. They can be played on a 3 string although you do have to have your slide working well. It's definitely worth the effort.
The classic John Lee Hooker riff in a Canned Heat tune from the late '60's. The main thing to get right with this one is your right hand. This is a great tune to practice those Hooker boogies, a shuffle with a s sharp staccato back beat.