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.
Wizards! The more more popular but non-niche tribes all kind of get stuck in their respective lanes of “mana, tokens, anthems” for elves, “tokens, sacrifice, damage” for goblins, “big flyer” for dragons and so on but Wizards resist such classification.
My favorite thing about Wizards is the versatility. You could be aiming for lots of creatures or just a few, a lot of small spells or a few big ones, interacting with the battlefield, everyone’s hands, graveyards, anywhere.
At any given moment you could be doing anything or responding in any way.