One of the best out there! RIP
The reason why his comedy worked was: he was ALWAYS joking when he insulted you. He didn't insult you because he was mean, angry, rude, or that he disliked what you did or that he hated you personally. He was joking about your quirks, insecurities, mannerism etc. And he wouldn't apologize about it either. Heard from many places that he actually was a very nice guy. He was even handed, he would joke about everybody.
Most of today's comics don't get that. They try to make jokes because their angry, or their trying to promote some agenda. Rickles had no agenda. He just joked about the general stupidity of everyday life.
He didn't like the insult comic label much either. 90 a good run!