The heart is the strongest muscle in the body. As an organ system however, ( like the skeletal system, or the endricin system etc. I'd have to say that the strongest is the central nervous system because it takes a licken every day, but keeps on working.
Logically the heart, since it is a muscle.
I have never heard of organs being described as "strong".
Its hard to tell because organs work in collaboration with one another.There is no one organ ca work alone.