Obvious slide greats would have to be Duane Allman, Elmore James, and Derek Trucks. I love that relaxed, hangin' out on the porch delta style so my current favorite's "Please Be With Me" I can't recall the band but Duane does the guitar work. It's on my Duane Allman Anthology record. If you're interested type Duane Allman - Please be with me into youtube you'll find it.

Another good one's the Little Walter cover Duane and Eric Clapton do of "Mean Old World."

If you wanna hear some roots listen to "Can't be satisfied" by Muddy Waters or search some Charlie Patton stuff (he's older than robert johnson so his recordings are often crude but there are some hidden gems forsure)

To end this rant on a positive note I'll mention that Rory Gallagher can rip a slide like it's nobody's business. When Rolling Stone asked Jimi Hendrix what it feels like to be the greatest guitar player he said "I dont know, ask Rory Gallagher."