My favorite thickener is a roux. It's similar to a beurre manie, except that it is cooked first. It also lends a great flavor what whatever you add it to. Also doesn't make your gravy clear/translucent like cornstarch or arrowroot.
My mother used flour . My grandmother used flour. I use cornstarch, but it doesn't reheat well . So when I know I'm going to have left- overs, I use 1/2 flour - 1/2 cornstarch . I like cornstarch best because it thickens clear, to flour's cloudy. And it's lighter.