I've had numerous interactions with Neal Adams over the years, often randomly, such as him calling me over to his table while I'm walking by or coming over to a booth I've been working, always just to shoot the shib. He's definitely a character and makes no secret of that fact that he's at a convention strictly to make money, but he's always been pleasant enough.
Don Rosa is yin and yang. Sometimes he's the most personable guy in the world, chatting you up and drawing sketches for free. Other times he acts like he'd sooner see you dead than say hello.
Jae Lee has always been crotchety any time I've interacted with him.
Two guys that are ALWAYS in a great mood: Stan Sakai and Steve Lavigne.