League of Legends (LoL) Question: Which champion should I play

First of all, good choices :). Midlane and jungle are the two most interesting roles to play. If you're interested in assassins, first I suggest sorting into AP and AD. Zed Talon and Yasuo are some of the most solid (although difficult to play) midlaner AD assassin picks. If you're more interested in AP, I would try Fizz LeBlanc Kassadin or Ekko. Unfortunately, assassins are very difficult to play, and they require a lot of practice to master. For jungling, I would recommend Master Yi. Mid-laner mains tend to favor him. Evelynn is a strong AP assassin jungler, you might like her. Shaco is also one of those assassin junglers. As for tanks, Sejuani Zac and Rammus are currently the tankiest junglers in the game. Zac also has a gap-closer: he's my favorite of the three. If you want a little bit of damage along with the survivability, I recommend Warwick or Xin Zhao. In the end the choice is yours- these are just some ideas that might help narrow it all down. Good luck!

I cant speak much for mid lane, but for jungle, i can recommend Maokai, Sejuani, and Amumu. They are all extremely tanky, if you build the right way, and both maokai, and sejuani both surprisingly deal some decent damage. Amumu is mainly based around cc, but is extremely effective when you build a Rylai's Crystal Scepter on him, due to the items passive, which slows an enemy if you hit them with any ability. This pairs very well with his Despair.

Hope this helps,

~moon

Assassins and Tanks in the Mid or in Jungle that I consider fun to play, regardless of their difficulty:

Fizz is the only Mid assassin I enjoy playing, though many people like Talon and Zed.
There aren't many tanks or off-tanks that play in the Mid Lane (usually you see them as junglers, top laners, or supports), but I really like playing as Swain (both pre and post his recent rework), and he can still put out respectable damage while building tank items.

In the Jungle, there are tons of assassins and Tanks that are fun to play here are some of my favorites:
Tank: Sejuani, Gragas, Skarner. My favorite of these three is Gragas, though the Jungle is in a really good place for Sejuani and Skarner right now, and Gragas isn't doing too hot.

My favorite assassins in the jungle are Shaco and Nocturne. They both take a bit of getting used to, but there are great guides for them on here. PsiGuard's Nocturne Guide and Whitecr0w's Shaco Guide

The best way to answer this would require us to know what champions you already kind of like, and for what reasons. Here's a quick list of my favorite champions that aren't too difficult to learn for each position (there are some I like more but have a steeper learning curve):

Top: Renekton, Illaoi, Wukong
Jungle: Warwick, Sejuani, Kayn
Mid: Lissandra, Xerath, Malzahar
Marksman: Draven, Lucian, Caitlyn
Support: Sona, Leona, Nami

Many of these champions have good range, easy burst, reliable escape mechanics, or a combination of the three.

List some of the champions you like, and why you like them. I'd be happy to make a suggested list for you.