Udyr and Skarner are great tanky picks, as are Shyvanna, Mundo and Nautilus. All have their strengths and weaknesses..
Udyr has great sustain in jungle due to turtle shield, move speed increase and stun with bear, and insane clear times with phoenix stance - great at counterjungling - good early ganks due to bear stun - no ultimate though
Skarner also gets a shield and mvspeed/AS increase, perma-slow with his q, and at level 6 his ganks are godly. Prior to 6 ganks are kind of meh unless the enemy is really out of position and you have time to get 2 shots of q off. Also good at counterjungling due to shield/AOE with q..
Shyvanna is supposed to be one of the best at counterjungling but her ganks are meh due to only having a knockback with dragon form - probably a good pick if your lanes have good CC
Nautilus is the CC king, but can be kind of slow in the jungler - easier to counterjungle than the others, and you need to hit those dredge lines to pull of ganks due to pretty slow mvspeed compared to others.. but if your team has their **** together you can hit ganks every time
Mundo has a great AD steroid, perma-tenacity and AOE damage with his burning agony, and a great slow with the cleavers (as long as you hit them).. after level 6 you practically have infinite sustain and free ghost with ult.. but hitting those cleavers is HUGE - good luck ganking if you miss with them!..