Your Brain. After that it depends on where and in what climate you are attempting to survive in, but most survivalists would probably agree that food acquisition implements (fish hooks, line, arms etc) are paramount, followed by fire making tools, proper clothing for the situation and certainly in the top most wanted group would be a good knife.
The single most important piece of safety equipment you can wear will depend on the activity you are participating in.
The ice skates is the most important piece.
The hockey stick
Depth finder (sonar)
A propeller guard.
There are many peices of equipment needed in soccer but one of the most important ones has to be shinguards.
I don't know what you mean by who have cleats. But the most important piece of soccer equipment is the ball. You can run drills without it but you can do that without any equipment. Other than that the ball is the most important. Cleats all depends on what type of surface you are playing on, turf, grass, indoors, on fire, or ice you get my point.
Printer
Abdomen guard
The abdominal wheel has gotten good reviews. Here is an example: http://ezinearticles.com/?Is-the-Abdominal-Wheel-the-Most-Important-Piece-of-Exercise-Equipment-Ever?&id=1917654 There are also many stomach/ab exercises that require no equipment at all that you can do at home.
The most important consideration when installing playground equipment is the finishing material.
Sabas equipment