The simple answer is this... whichever exercise/workout you're going to do. There's no point in identifying the most intense exercises and workouts if you despise them because the likely hood of sticking with such a routine is very small.
When we talk about weight loss, or fat loss, what we are really talking about is energy expenditure. The scientific community communicates this in terms of METS (metabolic equivalents). A MET is a measurement of oxygen consumed. Think of your energy system like a camp fire. In order to have a successful camp fire you need oxygen. Your body is the same way, calories need oxygen to "burn".
At rest the we consume approximately 3.5mL of oxygen per kilogram (2.2 pounds) of body weight per minute. This is considered 1 MET. The more MET's required to perform an exercise the more intense the exercise is.
Vigorous intensity exercise is 6-9 METs, and High Intensity is anything over 9 METs. The higher the METs the more calories being consumed. While these are beneficial to weight loss they will make you breathless and can be performed only in short bursts. Finding a HIIT (High, Intensity, Interval, Training) program that works for you can be an excellent part of a weight loss program.
--->LOOKING TO DEEPEN YOUR KNOWLEDGE OF EXERCISE, AND FITNESS? <----
A few exercise examples are: battles ropes (pictured above), jumping jacks, jumping rope, and bench hops. However, at the end of the day the best exercise or exercises are the one's you can do safely, and that you can commit to.