I'm not that creative when it comes to catchphrases but I choose a few of them:
Tex: "cool"
Flo: "ya heard"
Frank: "Crackers"
Gladys: "waku"
I'm a lot more creative with greetings however:
Frank: "Get off my lawn" "Hello young'n"
Flo: "Heya"
Flora: "I love you"
Goose: "Howdy"
Gladys: "Konichiwa"
Tex: "Cool it"
Sparro: "Dude"
Molly and Drake: "Quack"
I try to give them more personality this way. xD
Howdy is a good fallback for me when I can't think of a new greeting, especially for any villagers that seem a bit country western-esk