My favorite combinations are currently W/U and R/U, because of so many combos that I have discovered. Currentyly I play a slow W/U deck, and here are the following combos for W/U:

Note: If you attempt to build this deck also use a lot of little creatures(Especially Longbow Archer) and conterspells (Especially Divert from Odyssey)

(Walls can't attack.)
Like arguing with ice -Vodalian expression meaning "wasting your time"

Comment: I use four of these 3 mana walls in my deck since they are the best walls for Type II I have seen, yet.

Untapped creatures you control get +0/+2.
". . . Our castle's strength Will laugh a siege to scorn." -William Shakespeare, Macbeth

Enchanted land has "Tap, Sacrifice a creature: You gain life equal to that creature's toughness."
"The animal spirits are the Ancestor's grandchildren. Remember them." -Mystic elder

Comment: Usually I wind up with too much defense so I sacrifice my walls to get a life boost or I play Wayward Angel and Sacrifice my walls before playing it, to get Threshold and not to loose any Creatures.

Enchanted land has "Tap, Discard a card from your hand: Gain control of target creature until end of turn."
Politics is just a subtle form of mind control-and cephalids are master politicians.

Comment: basically this is an Uncommon which if used correctly is uncommonly damn good. I use it with Animal Boneyard and To get rid of anyoing creatures yearly in the game, make sure you have cards to discard, in my deck I had a littlebit of problem with it, I usually had Reverse Damage as a discarding card, and Reverse damage can be great to pretend you can not deal with a creature, let them attack, and then get a huge amount of life.

Flying Attacking doesn't cause Wayward Angel to tap. Threshold - Wayward Angel gets +3/+3, is black, has trample, and has "At the beginning of your upkeep, sacrifice a creature". (You have threshold as long as seven or more cards are in your graveyard.)

Comment: This Angel always wins me the game by putting Spirt link on it and having at least 2 Devoted Caretakers from Odyssey protect it. Once my Opponent is done killing all my devoted caretakes, I simply play Breath of Life. I use a lot of spells in my deck also to dramatically slow down opponents attacks with 4x Spirit Link, 4x Pacifism, 4x Pariah, and 4x Kitar's Desire. Usually this lasts a while until my opponets play tranquility which happens rarely.

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If you control a creature, damage that would reduce your life total to less than 1 reduces it to 1 instead.
"Believe in the ideal, not the idol." -Serra

Comment: Worship of course is not suppossed to last forever, so try using 2Auramancer and 3 Breath of Life like in my deck.

Comment: The above combo is kind of a late game finisher, but an Iridescent Angel can be really hard to deal with, sometimes, and believe me, with 3 Breath of Life in my deck, having the Angel back in play for less is usually a pain in the neck to my opponents. And I allways get to play this combo because of all my walls.