Maokai and Skarner are my personal favorites.
Maokai ganks are insane. I love rolling in and then waking them backwards with my q. I find him pretty epic during team fights. His ult is great, assuming you can keep people in it. His Saplings act as 35 sec wards. My favorite junglier by far.
Skarner is fun as well. His ganks are great, but not quite Maokai level. That slow is fun, and his ult if you get it is hilarious. I build him and Maokai about the same and find Mao a little tankier. Can't go wrong with either really.