I get this question often. It's usually followed by "...for getting rid of my belly/thighs/butt/tricep fat"... etc.
Regardless of what you are looking to accomplish, that question in its current form is unanswerable.
There are exercises that work very well for certain muscle groups and movement patterns, various forms of posture, cardiorespiratory training, etc. So while trying to create a "supreme exercise" is a futile task, I will devise a criteria by how I define a "great exercise"
One that incorporates big, compound, multi-joint movements that requires a degree of coordination and integration to perform.
For arguments sake, If I could only pick 3 exercises that I had to do for the rest of my life, they would be... DeadliftsPull-upsWind sprint intervals (90-100% intensity - Tabata style)
By definition, the above work massive amounts of muscle groups, require a large degree of integration and include a very high intensity cardio. Deadlifts - are the prototypical posterior chain exercise - working the muscles of the back of the legs, butt, lower back and require a serious amount of stabilization through the core and a fight to keep a neutral spine. Pull-ups - I confess I love them. All you need is a bar and you have a near perfect lat/pec/trap (mid/low) and an appreciable amount of stabilization through the core and shoulder girdle. Wind sprints - I love running and performing near-max efforts are a fantastic way to boost fitness in a hurry. Build up to these bad boys though.
What 3 exercises would you do if you could only choose 3?
Image Credit: Flickr/elvertbarnes