Pitting and Crevice corrosion are usually the worse.  However, I have seen in several cases that load bearing components may experience worse Stress Corrosion.   

The eight forms of corrosion are as follows:

(1) Uniform attack (or general corrosion)

(2) Crevice corrosion

(3) Pitting

(4) Stress-corrosion cracking

(5) Galvanic corrosion (two metal corrosion)

(6) Intergranular corrosion

(7) Selective leaching (dealloying)

(8) Erosion corrosion

Crevice and pitting corrosions are the worst

Any type of localized corrosion, particularly pitting (a localized form of corrosion by which cavities or "holes" are produced in the material) is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against.

Experimental findings in laboratories and corrosion case studies in industry revealed that pitting and crevice localized forms of corrosion are the most dangerous.

Pitting is the most dangerous and vicious of all other type of corrosion because its gradually and silently  destroying the metal completely. 

Although I do not disagree that pitting can be dangerous, I believe that stress corrosion cracking is the most dangerous.  Frequently the cracks cannot be detected until the object fails unexpectedly. Since stress is involved, failure can also be quite violent.  The danger is therefore a function of the inability to detect SCC prior to failure and the often violence nature of the failures.

There are cases where cracks were detected in an object prior to failure, but the degree of cracking could not be evaluated because the cracks can be so tight that they cannot be detected using any form of NDE.

Pitting without a doubt. The problem is that you don't easily see it as in uniform corrosion. You realize about it when it's too late.

Dear Nada 

The most dangerous forms of corrosion are:

pitting: since the material in many cases may be penetrated without a clear warning (because the pits often are narrow and covered) and the pit growth is difficult to predict.

Intergranular: because the cohesive forces between the grains may be too small to withstand tensile stresses; the toughness of the material is seriously reduced at a relatively early stage, and fracture can occur without warning. Grains may fall out, leaving pits or grooves

yes,Most of  localized corrosion  ar dangerous, especially pitting

Thank you.