On 2018's busiest day for global aviation, there were 66,000 possible international connections between flights arriving and departing at Heathrow within a six-hour window, a new report by UK-based air travel intelligence company OAG reveals.

OAG's Megahubs International Index 2018 pits the world's biggest and busiest airports against each other to discover which lead the way as both regional and global connecting points by comparing the number of scheduled connections to and from international flights with the number of destinations served from each airport.

Top in the US

Chicago O'Hare International Airport has moved up the rankings to second place, while third and fourth place go to European airports Frankfurt and Amsterdam.

Singapore Changi Airport -- voted best in the world six years in a row -- ranks highest in Asia (and eighth overall), but another Asian airport, Beijing, is the largest in the world in terms of scheduled airline capacity, or number of seats on flights.

However, a comparative lack of international connections means that Beijing ranks just 32nd overall.

Toronto Airport is the second best-connected airport in North America, and ranks fifth overall.

Los Angeles International (No.6), Hartsfield-Jackson Atlanta (No.7) and New York's John F. Kennedy (No. 16) are the other North American airports to make the top 20, while Mexico City -- in 20th place -- is the highest-placed Latin American airport.

France's Charles de Gaulle (No. 9), Germany's Munich (No. 11), Istanbul's Ataturk (No.17) and Dubai International (No.18) are the other highly placed entries from Europe and the Middle East.

Strong in Southeast Asia

Other than South Korea's Incheon (No.15), the highest-ranking Asia Pacific airports are all in Southeast Asia: Jakarta (No.10), Kuala Lumpur (No.12), Hong Kong (No.13) and Bangkok (No.14).

The index also reveals the key airlines operating out of each of the megahubs, with many of the top 50 airports having one airline that operates 40% or more flights.

At Heathrow, British Airways has a 52% share of flights, while at Hartsfield-Jackson Atlanta, Delta Airlines operates nearly four out of five flights.

Heathrow's domination is unlikely to be unchallenged anytime soon.

With already a strong lead over the competition, in June 2018 British lawmakers made the controversial decision to greenlight the construction of a third runway , which could increase Heathrow's passenger capacity from nearly 80 million passengers per year to 110 million by 2030.

This would make it, by current statistics, the world's biggest airport in terms of traffic.

A composite image shows 42 flights taking off from Heathrow in a one-hour period on November 2, 2016. Dan Kitwood/Getty Images Europe/Getty Images

The world's best-connected airports

1. Heathrow Airport (LHR): Connectivity index: 333. Dominant carrier: British Airways (52% share).

2. Chicago O'Hare International Airport (ORD): Connectivity index: 306. Dominant carrier: United Airlines (48% share).

3. Frankfurt Airport (FRA): Connectivity index: 302. Dominant carrier: Lufthansa (63% share).

4. Amsterdam Airport Schiphol (AMS): Connectivity index: 286. Dominant carrier: KLM (51% share).

5. Toronto Pearson International Airport (YYZ): Connectivity index: 271. Dominant carrier: Air Canada (60% share).

6. Los Angeles International Airport (LAX): Connectivity index: 257. Dominant carrier: American Airlines (21% share).

7. Hartsfield-Jackson Atlanta International Airport (ATL): Connectivity index: 256. Dominant carrier: Delta Airlines (79% share).

8. Singapore Changi Airport (SIN): Connectivity index: 253. Dominant carrier: Singapore Airlines (23% share).

9. Charles de Gaulle Airport (CDG): Connectivity index: 250. Dominant carrier: Air France (51% share).

10. Soekarno-Hatta International Airport (CGK): Connectivity index: 249. Dominant carrier: Garuda Indonesia (28% share).

11. Munich Airport (MUC): Connectivity index: 237. Dominant carrier: Lufthansa (59% share).

12. Kuala Lumpur International Airport (KUL): Connectivity index: 233. Dominant carrier: AirAsia (40% share).

13. Hong Kong International Airport (HKG): Connectivity index: 230. Dominant carrier: Cathay Pacific Airways (26% share).

14. Suvarnabhumi Airport (Bangkok) (BKK): Connectivity index: 230. Dominant carrier: Thai Airways International (21% share).