I never play mid lane, but, as preparation for ranked, I want to learn at least 5 champions in every position. What mid champions do you suggest? I already have Ahri. I was thinking Azir, but I have heard that noobies should stay off him. Cass and Viktor are also strong considerations. Any tips?
5 champions is probably spreading yourself too thin. To be honest, if its just to fill Ahri alone is probably perfect because she's a very safe pick.
To be diverse you'll probably want at least one siege mage, one assassin, one AD based mid.
Seige mages include , 
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Assassins, 
AD: See AD assassins, also 
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I do not main mid but when i have to fill i almost always go with pantheon mid. Usually your lane opponent is a squishy ap champion that you can poke out of lane. If they are ap, rush a hex drinker and asuming you havent fed and are at least even in cs the lane is yours. The trade off however for your dominant early game and mid game is you will fall off rather hard late game compared to traditional mids so you should focus on ending the game asap. Also pantheon mid has great roam potential with his ultimate.
orianna is fun and rewarding when you get her down
Ahri and Azir are my two main midlaners :D Suuuper fun
I also recommend Ziggs or Xerath if you find Azir too difficult to learn but you still enjoy the long range, sustained damage mage thing.
I really like Orianna but I have a hard time playing her in soloqueue. When I queue with my friends it's great and I get lots of Rengar + Ori ults off, but in soloqueue I find myself not really able to pair up with my team as well and nobody seems to understand what it means when I put my ball on them.
also you have to be super careful with orianna right now because she gets absolutely dumpstered by Azir and he's quite popular at the moment due to LCS :(
are my mid laners usually, i main Lissandra. all pokey lane bullies when played right. liss gets a free spell as her passive, ahri gets a free heal off a spell as her passive ( i recomend using it with Q) and xerath gets mana back on an auto attack for his passive. You can poke the enemy laner down a bit, push mins to tower and roam for a quick gank top/bot, or push to drop a cpl wards in the enemy JG. Just make sure you are aware of ganks when you push. a lot of ganks in mid are avoidable with proper warding. They key is to just find a few that you actually enjoy playing. dont always go with the popular choices. sometimes, especially in lower elo, you can really destroy a lane playing a champ that most ppl dont see. 
Syndra is an example as you dont really see her often and her kit is pretty hard to get used to laning against, especially when she starts tossing tanks at you.
Annie is a very strong midlaner with an insane amount of burst damage. She's like an easier version of Ahri, only with better cc, and a lack of skillshots. Annie was easily my favorite champion to play when I was under level 10 and she's an all out really underrated champion who dominates in lane most of the time.
Leblanc is my all-time favorite champion. I never get tired of playing her because there's just so much you can work with. Her kit is relatively simple and includes amazing early game burst, allows for solo kills without being ult reliant, has good escape, and a bit of cc. Though she is very mana hungry early game, she can poke easily and get close to enemies to pull off q-e combos or autos and immediately return to safety with her w. She has great roaming potential to help out other lanes and can snowball into a complete monster, once she gets her power spike. Only downside is how squishy she is, though normally you shouldn't be staying too close to the enemy for long anyways.
Katarina is always kinda iffy for me, since I don't really favor melees or staying close to the enemy, but she is a very strong champion that is banned almost all the time. Her ult is amazing in teamfights and she has great burst at level 3, if she can proc her marks from q. She's a very snowball-y champ and I rarely ever see a Katarina fall behind. If she does, she is still super OP and can deal out a significant amount of damage.
Lux is kind of a risky midlaner, but if she can get a lead, it's very rewarding. I hate laning against Lux because she has such high damage early on (I was so disappointed and angry when I found out the recent patch actually BUFFED Lux's abilities). She does really well as both a utility and burst mage, with great cc, a kickass shield, and a wide, multitarget ult.
Would not recommend Azir as he takes awhile to learn and master, definitely look into Viktor and Orianna in my opinion.
Ahri, Zyra, Annie, LeBlanc, Azir, Katarina, Orianna.
