Mastodon cover of Stairway to Heaven might be the only song I've ever heard that made me cry the first time I heard it. They recorded it as a tribute to their manager that died of cancer. It's beautiful and truly moving, it is my favorite of all time made by one of my favorite bands.
Social Distortion cover of Alone and Forsaken by Hank Williams is one of my absolute favorites.
My two absolute favorites are Sister Hazel's cover of The Cars' "Just What I Needed" (I love the slide guitar playing the synth parts from the original) and Lynyrd Skynyrd's cover of Merle Haggard's "Honky Tonk Nighttime Man" (Steve Gaines and Gary Rossington are absolute beasts on this).
"Well, if you can’t have a great season, at least ruin somebody else’s." - Olin Buchanan