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.
I’m stuck on Demons. It’s challenging to keep the mana curve to a manageable level but otherwise finding it fun to play especially after the Warhammer 40k set added some additional choices.