I can second Danny Devito being super nice. Met him once backstage at a concert, and another time at comic con. At the concert, he was the one who approached me and the crew I was working with after he seen us whispering about him. Said whats up to everyone, shook our hands, and asked us to take a pic with him on his camera. Dude from Queens of the Stone Age was the one taking the picture haha.
I've run into DeVito multiple times backstage at Coachella and we always have a great conversation about whatever band we're seeing, who looks good later that day, and music in general. He's always completely charming and engaging and bumping into him is something I look forward to every year.
My ex is a filmmaker/director, so I've had the opportunity to meet a few stars along the way...but the nicest guy EVER is Danny DeVito. He isn't the least bit stand-offish, and he let me use is trailer to nurse my daughter on the set of Jack the Bear. His wife Rhea Pearlman is sweet too, and extremely funny.
Also very nice? Drew Barrymore. She was in a screenwriting class with my ex, and invited us for coffee when I picked him up after class.
My dad ran into him in Chicago years ago when they were filming a movie. It was 102, hottest day of the year and my dad was taking a break from a long walk throught he loop on a bench, he looked and down at the other end was Danny Devito, he'd stepped a couple hundred yards off set to drink a huge thing of water.
My dad didn't want to bother him, they just sort of nodded and Devito gave a kind of "thanks for not harassing me while I'm working and sweating my ass off" smile.