I'm planning to cut into small cubes and place raw on the pizza before putting it into the oven. I wondering what cut of steak would be good for this purpose.
Any meat that is good for fast cooking (BBQ, frying, grilling etc.) will be suitable for this. Do not use meat that is normally casseroled or slow cooked. For beef this is the non-exercised top parts of the cow. Typical western cuts are called sirloin, rump, and fillet
You need very thin slices of meat more than cubes if you want any level of browning and cooking to be achieved
The easiest way to do this is to partially freeze you meat for one to two hours in a normal domestic freezer. A slab of meat with a profile of around 30 x 80 mm (±50%) is the easiest to handle. Use a sharp chef's knife or meat cleaver and slice it into 2 to 3 mm thin slices which will cook very nicely on a pizza
A well marbled (mixed fat content) cut would work better, like in this picture