Dude,
mechanical engineering is very broad and it's almost impossible to answer your question. But I guess I would say automobiles and electronic gadgets used in everyday life (computers, phones, medical devices). It all depends on where you land a job.
All mechanical engineers may not focus on same items. As a mechanical engineer I can tell you my taste that I believe is the concern if not all but of many. We love mechanisms! Like what you see in watch, gear box, Transmission etc. Even simple mechanisms like levers that operate to close car doors, petrol outlet opening, bonnet opening, seat adjustment etc. Don't they make you feel wonderful. There are many much more such things you will see in real life. Try to find how the mechanical movements happen in your daily life. None of these are comparable to electronic gadgets. But many electronic gadgets depend a lot on mechanical movements. Just open and see any Tape / cassette recorder, you will agree my point of view.
Mechanical engineering is a very broad topic. You can work on things from weaponry to air-conditioning and heating. It is very hard to determine exactly what the most popular branch of mechanical engineering is because mechanical engineers can branch out to almost any field and do almost anything that is related to engineering. For example a mechanical engineer has background in thermodynamics, statics, dynamics, and fluid mechanics so a mechanical engineer can go into almost any engineering field knowing at least a little bit of background.