What is the major fighter jet used today?

Answer 1

The highest Volume fighter in the use is the US Air Force F-16 Falcon. This single engine fighter is sold to several US allies. The next is the Twin engine F-15 Eagle, then the Navy A/F-18 Hornet including the recently upgraded E,F versions, the Super Hornet. The A stands for attack lending to its ability to also be a bomber. The Navy F-14 Tomcat runs a close third but is no longer in production. The Stealthy F-22 Raptor is fairly low volume. The single engine F-35, the Joint Strike Fighter, is just ramping up production and has Air Force, Navy, and Marine/Army versions and will be the primary replacement for the F-16, F-18 and A-8 Harrier/A-10Warthog aircraft, respectively. The aircraft is jointly produced by Europe allies, but production volumes are very controversial and uncertain.

Answer:

Very probably the MiG-21. Although largely overlooked in the West it's been built and exported in large numbers.

While roughly 4,500 F-16 have been built there were over 10,000 MiG-21s built in Russia alone, to this we can add nearly 200 in Czechoslovakia, over 650 in India and 2,500 - 3,000 in China.