My favorite format is limited, either sealed or draft. You never know what you'll get, and it forces you to play with different cards, rather than decks of 4x this, 4x that, 4x the other.
It also forces you to modify your playing style - beatdown decks aren't fast enough, and control decks don't have enough control. Combo decks are non-existent, except for minor stuff like Ancestral Mask + lotsa Enchantments.
Unfortunately, there are some broken decks even in limited. I once played a guy that opened 2 Lightning Dragons and 2 Fault Lines. He lost in the finals to a guy who opened Phyrexian Processor, Angelic Chorus, 2 Catastrophe and 2 of the White instant that gains you 2 life for each creature in play.
Overall though, I think limited is the format that requires the most skill.