Blue mainly, though with a splash of black. It's more the deck itself rather than the colour for me, but control and card advantage is just excellent. Blue/black decks are the ones I'm best at building though I'd say (even though my mono-green Elf deck is a powerhouse unless it's stopped early). Favourite deck I've ever built is a blue/black Millcontrol deck. Since I only play casual and we don't use sideboards it has to have a lot of dead cards for many matchups, but they're rarely gamebreakers.
I used to have a deck built around Mycosynth Lattice and March of the Machines, but it got banned from my group. Not because it couldn't be beat, just that it was so incredibly boring to lose to. If you're not familiar with the combo, it makes everything artifacts and all artifacts creatures with power/toughness equal to converted mana cost - all lands become 0/0 creatures and die upon entering the battlefield, removing nearly all decks main mana supply forever. I meanwhile had a bunch of mana-producing artifacts (with Darksteel Ingot the main threat - a 3-mana 3/3 indestructible creature that taps for 1 mana of any colour). Very vulnerable until March of the Machines was played though, as I had no real creatures.
I used to have a deck built around Mycosynth Lattice and March of the Machines, but it got banned from my group. Not because it couldn't be beat, just that it was so incredibly boring to lose to. If you're not familiar with the combo, it makes everything artifacts and all artifacts creatures with power/toughness equal to converted mana cost - all lands become 0/0 creatures and die upon entering the battlefield, removing nearly all decks main mana supply forever. I meanwhile had a bunch of mana-producing artifacts (with Darksteel Ingot the main threat - a 3-mana 3/3 indestructible creature that taps for 1 mana of any colour). Very vulnerable until March of the Machines was played though, as I had no real creatures.