If you were only allowed to do one exercise for upper body, what would it be? To answer this question, you would probably want to consider a movement that worked as many major muscle groups as possible, something you can do just about anywhere and that allows you to adjust the intensity.
Probably the best exercise to satisfy all of these needs is the pull up. Pull ups primarily work the lats (the upper, outer back) and the biceps but they also involve the rest of the back, the traps, chest, deltoids (shoulders) and forearms. No other single movement can work that many upper body muscles. You also don’t even need to belong to a gym to do pull ups, you can use an inexpensive chin up bar that fastens easily into a doorway, rafters in a garage, the branches on a tree, you could even rig up a bar or rope between any two sturdy objects.