If you want very strong wood, look for wood without knots. Knots significantly reduce wood strength.
This doesn't mean that wood with knots is very weak -- knots just reduce the strength.
Knots reduce strength because they are a change in the direction of wood growth. When wood grows normally, all the cells grow in the same direction, but a know has cells that grow in a different direction. Knots interrupt the cells, so the cells become weaker!
Wood grain also has an effect on wood's strength.