The most secretive part by faaaar is weapons for PAK FA. However, electronic systems comes close second. It is however surprising how open they have been regarding radar information. I can't imagine they were that open with AN/APG-77 way back in early 90's.
Btw, UOMZ kinda screwed up their systems on MiG-35 and Su-35S, and they were kicked from the later. Not sure if OLS is still UOMZ one on MiG-35, but lots of original intended stuff for MiG-35 is missing. Maybe Akril can fill us up on that?