Who is the highest ranking military leader in the Armed Forces?

For the United States, 3 answers, depending on what you mean. First, the President is constitutionally the Commander-in-Chief. Under the George W. Bush Administration, all other uses of "commander in chief" at lower levels was ended. The chain of command, by law, is through the Secretary of Defense to the commanders of the so-called combatant commands (e.g., the European Command, the Central Command). The Joint Chiefs of Staff (see below) can be thought of as off to the side, administering but not commanding the Armed Forces.

Second, the highest ranks currently found in the U.S. Armed Forces are General (Army, Air Force, Marines) and Admiral (Navy), i.e., the 4-star ranks, designated O-10 (i.e., 10th pay grade of officers). There is a "secret" "4-1/2" star rank, sometimes designated O-10S, for the commanders of the combatant commands and the chiefs of staff. U.S. law establishes 5-star ranks - General of the Army, General of the Air Force, and Fleet Admiral, which can only be granted in wartime. The last 5-star, Omar Bradley, died in 1981. There are historically higher ranks, General of the Armies (held by Pershing) and Admiral of the Navy (held by Dewey) but these have no established pay or devices.

Finally(!), the most senior actually serving officer in the U.S. Armed Forces is the Chairman of the Joint Chiefs of Staff, currently 4-star Admiral Mike Mullen.

For the United Kingdom, H.M. The Queen is the head of the Armed Forces, but in name only. The most senior officer is the Chief of Defence Staff, currently Air Chief Marshal Sir Jock Stirrup, who is equal in rank to a 4-star U.S. Air Force general. The highest ranks established for the British armed forces are Field-Marshal, Admiral of the Fleet, and Marshal of the Royal Air Force, but these are now to be granted only in wartime.

For Canada, the nominal head of the Armed Forces is the Governor General (who indeed is styled "Governor General and Commander in Chief of Canada") but the most senior military leader is the Chief of Defence Staff, currently an Army general, equal in rank to a U.S. 4-star general. General and Admiral are the highest military ranks established in Canada.

In many parliamentary democracies with the chief-of-staff system that Britain and Canada use, the question of who's in charge is a bit muddy. The head of state is often the nominal head of the forces, but the prime minister and defence minister are really in charge.

But many countries, especially outside the developed countries, have a commander-in-chief system where some general or admiral or air marshal is indisputably in command of the armed forces.

For ranks, you can peruse www.uniforminsignia.net, and Wikipedia has quite a few accurate articles surveying the subject - start with "Field Marshal".