いきなり頬を張り飛ばされたこっちの方が、よっぱムカついてるっての I think not”よっぱ”. Probably”よっぽど”? 1)張り飛ばすis the original form of 張り飛ばされた which means slap with your hand or knock you down. This is ~された, so it becomes past-tense of passive voice. 2)こっちの方が~ usually means "this is more ~." But in this sentence means "My situation is more~." For example, if someone says "仕事忙しかった！/The work was busy!", you can say”こっちの方が忙しかったよ！/My situation(in this case 'work') was busier!” You separate words incorrectly. So I changed for you!xD 3)よっぽど＝(Something is) way more. And usually this is set with 2) For example, ”英語の方がよっぽど簡単だよ”、”仕事する方がよっぽど大変だよ” 4)ムカついてる of verb form is ムカつく which means get irritated or mad. ムカついてる is adjective form means irritated or mad!xD 5)We use ”っての” when you are really angry or frustrated from the bottom of your heart. For example, imagine you have to read college level Japanese by tomorrow and you can't understand any of those for three hours. You must be getting tired, mad, and frustrated to it, right? And, someone asks "まだやってるの？/You are still doing that?". Then you'll get angry and say"めちゃくちゃむずかしいっての！" Usually something that you don't have to say it but you suddenly say it with an anger. いきなり頬を張り飛ばされたこっちの方が、よっぱムカついてるっての So, when all combined together, it becomes like this. I, who have been suddenly slapped my cheek, am more mad (compare to you). If you still don't understand, feel free to ask me again!xD

Sorry for the typo... I was too tired. The title should be: What do こっちの方 and ムカつく mean?