Every class is good, it boils down to play style.
Druids have massive EH but slightly lower avoidance levels. they have their own "block" mechanic in the form of savage defense, and have barkskin and berserk. Their primary AoE tanking tool is swipe which was buffed to be a 360 degree attack around the bear and is spammable.
Paladins have very good HP scaling, good avoidance levels, have a guaranteed block mechanic, have very reactive tanking in that a lot of damage is generated from being hit (holy shield) or just dropping a consecrate, which is part of why their aoe tanking abilities are considered paramount. Likewise they have divine shield, and Ardent Defender as cooldowns. I'm not sure how much Lay on Hands comes into play as a tanking cd, but I suppose it might be used as such?
DKs have decent HP, high avoidance levels, but no block mechanic. They have a very unique style of play in that they have runes, and then runic power (think energy that becomes rage). They've seen a lot of changes being the new class and many people will tell you they're OP, or they suck, or what not, but the truth is, in the hands of an experienced tank, they'll do well in just about every situation (except many many small attacks rapidly in succession, something block mechanics excel in, and something blizz has said they'd like to address for DKs). All 3 talent trees become viable tanking specs, which allows lots of different flavors of tanking within the class itself, and each spec has it's own pros and cons, and with cooldowns all their own.
Warriors are your middle ground, they don't have the highest HP scaling, but they have a ranged slot and the highest base HP to help balance out the HP differences. They have very good avoidance levels (basically the same as paladins) and have spikier block mechanics in the form of shield block (100% block rate and block value for a while, every 40 seconds). Warriors are the kings of mobility with warbringer, and are often enjoyed by players because of the activeness in warrior tanking (there's always a million buttons to be pressed). They may not be the best in aoe tanking, but they are by no means "gimp" at it, it just takes a little more effort, and they have the best burst threat abilities of the 4 classes (thunderclap + shockwave). They have SOME reactive tanking abilities in the form of damage shield, and have what many consider the "baseline" tanking cooldowns many of the other tanks have been balanced to have: shield wall and last stand.
The best class for a tank, is the class YOU enjoy the most, and are willing to put in the effort and time to learn how to play, to gear up, and to stick by even when the QQ is overwhelming, or when the naysayers are knocking on your door. Every class is viable, and although there are variances in their ability to do various things such as AoE tanking, survivability, threat, dps, etc, at the end of the day, the choice boils down to you. How to use each tank effectively requires you to pick one, play it, and read the various guides we have all over this website.
I'd like to take a quick note to state that this seems like a legitimate question, and devolving this thread into an x class vs y class v z class ZOMG OP, QQ thread will probably quickly lead to the closure of the thread as well as stricter than normal infractions being doled out. Input when constructive is good, input when destructive is not. Keep that in mind before you post.