Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE FLIGHTS (DestAirport VARCHAR, SourceAirport VARCHAR)
TABLE: CREATE TABLE AIRPORTS (AirportCode VARCHAR)

### Question ###
What is the code of airport that has the highest number of flights?

### Accurate SQL ###

SELECT T1.AirportCode FROM AIRPORTS AS T1 JOIN FLIGHTS AS T2 ON T1.AirportCode = T2.DestAirport OR T1.AirportCode = T2.SourceAirport GROUP BY T1.AirportCode ORDER BY COUNT(*) DESC LIMIT 1