It seems like a good number of air forces have kept modernizing and upgrading their Mig-21s. My question is, how good are they now? How would they fair against a more modern aircraft, like an F-16, F-15, Mig-29, or Su-27?

Really very few have on the level the Indian Air Force has. Its still a small fighter hard to see and pretty quick so careless mistakes can be made against it.

Most good trained pilots would stay out of the Mig-21 few strong areas and win. Once going into military power the jet has a short air time just doesn't carry enough fuel.

The Cope of India and other training/mock fighting is done in a set place and time forcing both aircraft to be in a certain part of the sky thus giving a chance to take a shot at a F-15C. Somebody correct me but I cant imagine the Mig-21 having any kind of staying power.

Originally Posted by benaakatz

It seems like a good number of air forces have kept modernizing and upgrading their Mig-21s. My question is, how good are they now? How would they fair against a more modern aircraft, like an F-16, F-15, Mig-29, or Su-27?

As far as I know, they can be used as training and grand strike. There's no good or bad airplane but proper or not proper one

Serbia uses about 11-20 modernised MiG-21's, and im pretty sure they would be obsolete against an F-15/22/35 or a Su-27/30/33/35 and a MiG-27/35

They would be pretty obsolete against F-16s.