Oh, ancient history...
I haven't picked these up in at least 4 years.
I used to play a lot, but I simply can't find time for it anymore, and neither can the guys I used to play against.
For multiplayer I had a really solid Green-Blue combo deck, utilizing the react-capacities and flying skills of the Blue for the early game (to quietly grow) and the heavy creatures and unit boosts of the Green to finish it off...
Single Player:
Red-White combo deck, a la "White Heat"
All Black Necromany/Leech-based deck
5-Colour Sliver deck. This one was actually pretty decent, but later I removed the Red Slivers and it was even better. The key to a good sliver deck is, IMHO, having enough of the really good ones (Winged Sliver - Blue, Horned Sliver - Green, Cristalline Sliver - Blue/White, and one other that I can't seem to recal right now. The Sliver Queen was nice too
)