What is the least tender cut of steak ?

Probably chuck or brisket, although they are more for braising than cooking like a steak.

Cuts of Steak. This is the least tender section of the three. The short loin produces the T-bone, top loin steak, tenderloin, and the porterhouse. The sirloin gives the sirloin steak and the top sirloin. Other steaks like the chuck, round and flank steak come from those respective areas and tend to be tough cuts of meat.

Meat from the cows rear-end has always been perceived as the leanest part and most tender. I think sirloin would be the least tender but this has also more flavour. 

Probably chuck or brisket, although they are more for braising than cooking like a steak.

After that I'd probably say london broil.

It is sirloin ................

Almost any steak from any cut if it is not cooked properly can be tough.  A Steak cut from the tenderloin, one of the most tender cuts (NOT the most flavorful though) can be rendered almost inedible if cooked to well done.

Flank steak or Skirt steak can be pretty tough but if done properly are tender flavorful cuts. 

Chuck steak or brisket if not cooked correctly

Filet Mignon; tougher than leather. If you want a great cut, i suggest , Chuck or Shank... super tender. Yum