Having tanked as every tanking class available (although only as frost dk so far), I thought I'd list my top 5 favorite tanking skills. For dk's I've only looked at their base set of skills, the ones any spec have at their disposal.
5 - Swipe - Druid
Although not so awesome at lower levels, this becomes the instance killer at level 80 (with some decent gear). Swipe makes druid the only tank class who can constantly threat while on the run, making it possible to actually never stop (except perhaps for boss fights) in a heroic instance. Being what heroics have become, something to burn through as fast as possible, swipe is definitely the best tool for the job. The other tanks come in close having skills like Blood Boil, Hammer of Righteousness and Thunder Clap, but they just don't cut it quite like Swipe. For making Rocket Bear Runs possible, Swipe gets into position number 5 in my list.
4 - Hand of Reckoning/Faerie Fire Feral - Paladin/Druid
They're not exactly the same, but the most important part about them are. They do ranged damage. And they only have 6 second cd. Being able to pull something with an instant ranged damage, or just keep threating on something standing a bit away, is something I really miss when on my warrior tank (dk's can use Icy Touch, but since it uses a rune which could be on cd it's not as good). For making ranged threat during combat possible, HoR and FFF get into position number 4 in my list.
3 - Challenging Shout/Roar - Warrior/Druid
Beside some difference in names, these skills do exactly the same thing, pointing out the similiarities between warriors and druid. It's something as awesome as an aoe taunt. Although paladins and dk's rarely need such a skill because they have really cheaty aoe-threat skills instead, even they get into situations sometimes when you just think "if only I had an aoe taunt right about now". Because when you need it it comes like an angel from above. Funny sidenote though, Shout costs 2 rage (talented) while Roar costs 15 rage. Since you need to use the skill when nothing is hitting you, you probably won't have 15 rage to use it as a druid, which is something I mentioned here. Blizzard are funny that way... For being the savior of the day (mostly for warriors), Challenging Shout/Roar gets into position number 3 on my list.
2 - Death Grip - Death Knight
Many people would probably have this as the number one skill on this kind of list. Not only is it extremely handy, it is fun to use too! So much fun in fact that non tank dks use it all the time! And never on a caster mob! And even if the mob is already in melee range! Ahem...
When used properly this is definitely one of the best tanking tools in the game. It does exactly what every tanks wants any skill to do, position the mob exactly in front of you, ready for you to wreck havoc in their face. If you're lucky, you can actually pull a whole group with Death Grip. Like the mob packs on the second boss in ToC. Nice huh! It does get a small minus for being available to every dk out there who hasn't got a rat shit clue about how to use it though. For being one of the most fun tanking skills, while also being extremely useful, Death Grip gets into position number 2 on my list.
1 - Avenger's Shield - Paladin
Yes, here we have it. The skill I like the most while tanking. You might be abashed that I've picked this as number one over Death Grip (or some other skill) or you might agree with me. But any tankskill is used for the purpose to get as much threat on as many mobs as possible. Death Grip is awesome sure, but it only gives threat on one single mob. Avenger's Shield on the other hand, gives threat -and- silences three mobs. It is the pulling tanks dream, not having to worry about the rest of the group when pulling the first mob. A group of mobs rarely constitues of more than four, and being able to threat on a majority of those with an instant, silencing, ranged, cheap (considering tankadins never run out of mana) skill is almost too good to be true. Some of you might think D&D is as good, but it doesn't silence and it definitely isn't cheap. The only thing that differs D&D from Thunder Clap, Swipe or Consecration is that it's ranged.
For being everything in one skill I put Avenger's Shield as my top one favorite tanking skill!