When Cindy climbs up on my tummy/chest, looks me in the eye, kneads and purrs. When she's sleeping and snoring, I pet her, and it turns into a really loud purr.
When Swanie meets me at the door, gets comfortable on whatever I brought in with me (mail, lunch bag, etc.), and purrs his head off while I pet him. Or when he follows me around the house and "helps" me with my housework.
When they chase either other around the house, or play bat the cat around the bathroom door (one on each side of the door).