Dr. Strangelove, I was introduced to him in Blazing Saddles where he was the perfect straight man for Harvey Korman and Cleavon Little to riff off of. Plus, who else could say "Mr. Lamarr, you use yer tongue purtier than a twenny dollar whore!" with a straight face and look so earnest about it?
Pickens was also one of the most memorable parts of Spielberg's 1941, which isn't saying a lot, but he was up against a lot of good comedians in that one. He's the guy the Japanese sub captures, who swallows the compass from his Cracker Jacks to impede their invasion of California. And I think when he pretends to be taking a massive dump, it was perhaps the highlight of the film.
According to Wikipedia, Kubrick wanted him to play Dick Halloran- the role that Scatman Crothers took and would eventual cry during, after 100+ takes- but Pickens refused unless he was contractually obligated to do fewer than 100 takes! It sounds apocryphal, but who knows. Sam Peckinpah liked him as well, and used him in The Ballad of Cable Hogue, The Getaway and Pat Garrett and Billy the Kid. Pickens had a very likeable face and classic acting chops where he could express volumes with a single expression. He died in 1983, and it would have been an honor to meet him.
Real name Louis Burton Lindley, Jr., he was a rodeo performer to start, and took his stage name because he was told rodeo work would only pay "slim pickings." He was nonetheless inducted into the Western performers Hall of Fame. His final film was the '80s cult classic The Howling by Joe Dante, who cast such a recognizable and out of place Slim to keep the tongue in cheek manner of the film. It's one I haven't seen in years, and I think it's going in the queue in Slim's honor sometime soon. Happy trails, Mr. Pickens.