Bottom line...
In summary, at the risk of sounding un-aviation like, the Critical engine is the one, that should it fail, it generaly creates the most asymetrical yawing tendencies, creates the most drag, puts the most damper on performance...and requires the pilot to put the most opposite or correcting imputs to deal with a critical engine failure.
If you use this analogy, every aircraft has a crtical engine...even jets due to the rotation of the engines, slipstream affects..
IMHO...the critical engine is really the one that the PILOT has the most hard to time with...like some people deal with right crosswinds better then others, or if the left engine is out, they just turn better to the left, then they do to the right..
This might be splitting hairs, but planes don't fly themselves, pilot's do.