This ^

He counterjungles by simply walking into the enemy jungle and beating the **** out of the enemy jungler. Low health? He'll just walk off into his own jungle only to appear full health again 1 minute later.

And about the high single target damage, it might not be the best for clearing own jungle, (with blue and constant Q's still pretty fast) but it's THE BEST for stealing the enemy jungle, where you just want that important one down fast, not to mention it enables him to do the dragon extremely early. He just needs the wriggles and then it's byebye dragon.

Honestly, one of my favorite junglers is Volibear. He is a beast.

Camp Clearing: His E gives you three seconds of feared creep, and his W is like a mini-smite. He can clear really fast and in almost complete safety.
Ganking: Throw, then slow, then slash. If you have anyone even half-competent in lane, it's almost always a kill. Plus, it's way fun to watch people try and run from you when you come running out of the jungle
Counter-Jungling: He jungles quickly, he has an escape in his speed boost and slow, and his passive makes him SO hard to kill (especially 1v1). Not much more you could ask for in a counter-jungler.

Now, obviously, everybody has their own opinion on who the best jungler is, and I'm not saying that Voli is THE BEST JUNGLER, I'm not sure that any one champion can hold that title, but he is certainly my favorite, and I think I have good reason to think so :P

Camp Clearing: Shyvana/Udyr
Ganking: Twitch
Counter-Jungling: Shyvana/Udyr/Lee sin
Jungle Duelist: Trundle

Camp Clearing: Shyv / Dr. Mundo
Ganking: Nocturne, Shaco, Lee (Ganking mostly is dependent on gap closers and CC, any jungler with either of those can be good, Fiddle is a great ganker, with a gap closer and a CC, wether he's viable or not is another story however)
Counter-Jungling: Shyvana / even Udyr, dueling him is hard.

It all depends on your team's composition.

Skarner has great ganks both before 6 and especially after. I have found that his camp clearing is actually faster or comparable to shyvana's. His shield keeps him high health, increases his move and attack speed and he has innate life steal. Once he is lvl 6, if you can get a good flash and supress it is almost a certain kill. I run move speed quints which add to his ganking ability and speed in jungle. His crystal slash will massacre people early and even harder after sheen. Not only that but his passive allows for him to gank with his ult, go farm and return to gank with his ult only 30 seconds later.