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 love Elves. Just splurge them out on the board and give them all +20/+20. Sadly they don’t work well in Modern anymore. I also cut Natural Order and Craterhoof from my Commander Elves deck since it was too oppressive.