Repeating what is said in any number of summary posts almost verbatim is not the most helpful thing in the world.
Personally I have tried over 15 specs starting with Unholy (Pre-Patch 3.0.8). All of them are effective at tanking. Key word is effective - by that I mean viable. Most raids have 2-3 tanks so filling that role can be different on different fights.
Unholy seems the best for buffing raids with valuable (Increased movement is worth its weight in gold imo) buffs. It has an increased effect on multiple mobs and one AoE pulls its can put out incredible damage.
Unholy seems to favor casters with its buffs (+13% Magic damage)
Second highest health buffs
Blood seems the best for single target and has highest health of the 3 specs. Also provides valuable buffs to melee that stack with other similar buffs. Highest expertise through talents. Very good survivability.
All the trees have their high points and low points. Its a matter of play style and personal opinion. Any good player can make any spec work(To that end I mean barring gear/computer/lag/broken arms).
Please this post is a general statement if I left anything out its not to favor any one tank spec over another.