I haven't played in a while (because 10th. ed ruined the game for me, although I'll still gladly play with anyone, who agrees to play with pre 10th. ed rules), but my favourite colours are black and blue. I voted other, because I prefer two coloured decks.
I have a blue/black artifact deck (Ornithopters FTW), a blue/white Merfolk deck, and a green/black/white Fungus deck. I don't usually like green that much but the Fungus+Saprolings thing is just too awesome to not like.
The only colour that I've never really played with is red.