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 really like playing Knights, the entirety of the lore in the game around them is really fun too. Other than that I think birds or 1/1 flyers tribal is fun, as well as defender tribal with Arcades as the commander.