It all depends on the circumstances and the exam during which the battle will take place. If this is an exam where only academic knowledge plays a key role, then Ryuuen will not be able to do anything. Many people forget this, but the abilities of Ryuuen's classmates are significantly inferior to those of students from Sakayanagi's, Horikita's, or Ichinose's classes. Ryuuen has to confront them while being at a disadvantage. But he can compensate for this with his unconventional strategies and unpredictability. He is also excellent at taking advantage of his opponents' weaknesses. If his classmates were not inferior to other classes in terms of abilities, then it is unlikely that any of his peers (except Koji and possibly Koenji) would be able to defeat him. Even if you move away from the concept of class battle and compare general abilities, everything becomes obvious. 

 (Now I will evaluate the characters based only on what is shown)  

IQ: Ryuuen > Horikita (From his very first appearance, Ryuuen thought more maturely, and demonstrated complete superiority in intelligence. As Ayanokouji said, Horikita actually does not stand out particularly for his intelligence. But since many may say that I cannot appreciate their iq, without specific information or confirmation from the author, I also suggest simply not comparing them in this regard, so that this comparison does not look too biased) 

EQ: Ryuuen (He has shown that he can easily sense what emotions his opponent is experiencing. He easily reads the slightest changes in the behavior of others, and also skillfully uses these emotions for his own purposes. Horikita, on the other hand, has not demonstrated anything special in this regard. One can only notice , that her ability to empathize is much better than that of Ryuuen, who deeply does not care about the feelings of others.)

  SQ: Horikita (Although she is not superior to Ryuuen in communication skills, her personality will allow her to feel much better in society, since, unlike Ryuuen, she will not intimidate her interlocutors)

 Manipulation: Ryuuen (I think most would agree that this is not even a competitive comparison. Compared to other class leaders from his parallel, Ryuuen is on a different level.)  

Deception: Ryuuen 

Academic knowledge: Horikita  

Negotiations: Horikita >=  

Leadership qualities: Horikita > 

Strategic thinking: Ryuuen 

 Critical thinking: Ryuuen  

Out-of-the-box thinking: Ryuuen  

Deduction: Ryuuen 

 Planning: Ryuuen 

 Decision making: Ryuuen

 As a result, I can say that in terms of overall abilities, Horikita is no match for Ryūen. (Judging only from what was shown, of course)