Mass cloaked wraiths can catch almost any opponent off guard. Recommended for $$$ maps.

-After you have decent defense, build 6-12 starports, 2 armories, and a science facility.
-Build control tower on all of them, and upgrade air weapons and air armor.
-Research cloaking and wraith energy
-When you have enough minerals, build a wraith in each Starport. Keep building wraiths, supply depots, and keep on upgrading air weapons and air armor. Also build 2-4 science vessels.
NOTE: This requires many resources, and like I said earlier, this strategy is recommended for $$$ maps. In order to get a flow of wraiths coming out in each starport, you need around 24+ SCVs mining, and 3-4+ refineries.
-Once you have maxed your supplies (200), hotkey each group of 12 wraiths. You should have at least 9 groups, plus 2-4 science vessles.
-Cloak them, and attack your opponent, and bring your vessels with you. If he has surrounded his base with anti-air (missile turrets, spore colonies, etc), just ignore them and bust in.
-Go for the workers, and after they are dead, go for any non anti-air detectors (comsat stations, observers), and kill everything else in his base.
-After everything is gone except anti-air towers (and the things around them), send in some marines, goliaths, or tanks to clean up your opponent.