Can be any format.
For Commander decks, I have at least 5 Dragon tribal decks already and am planning to try to build a dragon tribal deck out of every possible multi-color combination plus mono-red eventually. But Slivers and Eldrazi are up there too as two of my favorite tribes to build around.
My favorite tribal decks before I took my EDH decks apart were vampires and dragons. Vampires for the aesthetic + flavor and having enough to go deep on the tribal theme. Dragons because who doesn’t want to whirlwind slam a deck full of fatties?