Yzerman's absolute peak would be in 1989. Sakic likely around 1996 playoffs or 2001 season. I'd take both of their peaks over Malkin's. He's a big guy and has a great skillset, but he's not a great two-way player like Sakic was in his peak and Yzerman in the late 80's reached a higher offensive peak than Malkin did in his prime, with nobodies for linemates.
Also, does nobody honestly care that Malkin can't win a faceoff and has never been considered a good two-way player? In today's NHL, that should be more important than say Gretzky or Lemieux not being Selke candidates back in the 1980's.
Malkin's level of play on 11/12 is damn close to Yzerman's level of play in 88/89. Both had dominant PPGs vs. their "mortal" peers.
I would put Malkin's offensive peak/prime level of play as being of superior quality to Sakic's and longer than Yzerman's.
As for the bolded, if Malkin needs to be better defensively given he plays in "today's" NHL, it is only fair then to lower the importance of Yzerman's and Sakic's defensive efforts given they played at a time where it wasn't as important. You can't have it both ways. I.e. we would hold Sakic and Yzerman to the same standard if they played today and critique them accordingly.
As for playoffs, I would rate them Sakic, Malkin and Yzerman.
All things considered, Malkin may have simply missed the chance to put up more full seasons in his prime to match/surpass these two but I don't think it is unreasonable to view him as being the most inconsistent player of the three.