There are so many things, but one thing per cat -

Cato: Usually standing on the bed, one of his many vocalizations - "Raa." Not a long, drawn out sound, just a small, quiet exclamation. ("Ra" as in "rabbit" or "radish.") Sometimes it's "er-raa-h or "mraa." Cute!

CB: His adjustability. He likes to get under the covers and allows me to position him for my comfort and his, although he doesn't tend to stay as long as he used to.

Serena: When she lays on her back with a curl to her body and head, loudly purring for a belly rub. This is amazing in a way as she can be a very fussy, aggressive cat.

Koji: When he looks up at me with his gorgeous blue eyes, then rubs on my leg and follows me around. This occurs in the kitchen, mostly.

Miette: I'll give one of her routines. On the bed, she'll look me in the eyes while walking over to me looking like a little panther, knead my stomach in quick strokes, then lie on my chest and purr for a little while, though I'd like her to stay longer. Then she'll get off and lie next to me on her back as an irresistible invitation for a belly rub.