I have been playing Mid Lane for a quite a long time,and I have technically played all the mid laning champions. Therefore,regardless to the price,I will tell you the Top 4 viable/powerful APCs in the whole entire game.
I have analyzed that if you are level 14,you will know the basics of the game and know what to do with new champions,so if you are confused,please let me know.
Gragas: Gragas is an extreme mage with a powerful burst with the mixture of great sustain for both mana and health with his W(which gives him extra mana and ignores percentage of the damage inflicted),and his passive,which regenerates percents of his health everytime he casts a spell. His Q can wave clear so well,while staying far away so you cannot be poked. His E can be abused to escapes and stuns,or even jump through walls with it. Although his E have been nerfed to 12 cooldown from 7,everytime he collides with something,his cooldown is halfed(6 sec,so technically shorter),and it deals ALOT of damage also. Grages' ultimate can deal so many damage,while making the people within the explosion rate fly away. I would cast my Q between you and the enemy(Q is a barrel which you throw and you can press Q again for it to explode),and ult to the enemy so he/she will fly towards you. You E into the person,so they get stunned in in the Q zone,and you blow your Q,then if they are not dead already,the boost of W(extra AD) will be enough to finish the enemy off.
Orianna:Before explaining Orianna's kits,I must warn you that Orianna is one of the most hardest champions to play in LoL,since she requires a lot of smart casting and timing. Orianna is scary. Not only does she deal a lot of auto attack damage with her passive(which deals addition damage with her auto attack based on AP and additional magic damage),her orb can be abused to destroy an entire team into a defeat with a single,successful smart cast combo. Orianna's Q sends her orb out to the designated location,it deals damage while its moving,and deals AoE in the final spot. W deals a small range,but extreme AoE damage and being slowed while in the radius,and the allies get sped up when they are in the W zone. Orianna's E can be cast on her allies or herself,and the orb will shoot back to them and shield them(quite a lot),and while coming back,the orb deals damage to people in the way. Orianna's ultimate shoots out a big AoE stun/damage which deals so much damage late game,that a single ultimate will turn the tide of not only a fight,but the entire game. You use Orianna by sending out your orb with your Q,and bringing it back(dealing more damage while moving back to you) with your E. When you wish to use it on a champion for poke,add W with Q to deal more damage(or even use ult if you feel that a kill is necessary). However,it is important for Orianna to keep her distance from the enemy,since she is very squishy and the good people will make sure to kill her before beginning a fight. Also,Orianna's orb is her main source of damage,so auto attack is used for taking off chinks of health left,or to kite while scaring them off with the massive auto attack damage.
Fizz:Fizz kills almost everyone,lets start with that. Fizz is extremely bursty at late game that it usually takes Fizz 5 seconds to kill a person with his combo. The best part of Fizz is his E. Not only is a perfect escape,and wall jumper,it makes Fizz invulnerable while casting it. Therefore,you can cast it in Nunu ults or other projectile ultimates and take no damage. Also,E can be used again to land on a designated spot,while dealing AoE(perfect wave clear). Fizz's W allows him to deal percentage damage,so tanks will have a very hard time with him. Fizz's Q gives him a nice dive with a lot of damage. Finally,his ultimate shoots out a fish,which will begin to rotate around a champion if it is nearby. It creates a little water zone,which slows every enemy down. After a moment,a huge shark eats the person,dealing DAMAGE. Also,when you flash or escape when you're caught by the fish,the water zone travels with you,so once you're caught,there is no escape.
Well...there you go! Hope it helped,and always reply to this answer if you want to get more info. GLHF and Don't troll