The Su-27 is SURE A WHOLE lot more modern thatn the Mig-21. And, sure, the Su-27 is of the same generation as the F-15, sort of, but aerodynamically it is way advanced over the F-15.
Can't say the same for the weapons systems and engines. There we have the edge and that alone in my opinion. An Su-27 or a later version like the Su-35, equipped with western engines and armaments would be tough to beat.
Look at the Korean fighter procurement going on right now. In my opinion if the Koreans could have got agreement from the Americans to allow the Russians to equip the Su-35 with westerns arms and engines, the competition would have been over in a heartbeat. But one has a rather hard time imagining the set of circumstances that would allow THAT to happen.
Isn't the Su-27 the Sov aircraft reverse-engineered around the Grumman F-14A that the KGB got an Iranian pilot to fly over to them, and thus picked up the sobriquet "Tomcatsky"? IIRC they copied the F-14's look-down, shoot-down radar whose loss compromised our ALCM's and Tomahawks.
I never said that the Flanker was even comparable to a Mig-21 or even a Mig-29 for that matter. But I don't want to overestimate the capabilities of PLAAFs Su-27s. Btw, I do believe that the F-15(K) has won the South Korean F/X contest. The French are kinda pissed off....