Lol, I've only played casually with my siblings, besides the few times I played some un-official sorta casual tournaments at the bookstore, (which is closed now ;_ so I wouldn't consider myself a competitive player, even though I do try to make good teams because my brother is pretty good (he attends official tournaments from time to time, and has won a good amount of loot from drafts) and hard to beat.
Oh, well I guess by format you mean the type of thing your deck does? For example, one of my decks, also my favorite, is a vampire life drain deck which uses black vampires that use lifesteal, and it's loaded with lifesteal cards, and pretty much anything that benefits mutually from vampires, or has life drain. I guess that's what you're talking about when you say competitive format? Another example would be my sliver deck, a deck with TONS sliver type creatures that benefit from multiple stacks of cards that give "all slivers get [insert buff here]"
But yeah, those are my 2 favorite teams I like to show off. And I might add that 54/60 of the cards in my sliver deck are holographics xD