Is the metric sytem the most reliable?

That is the only convertible system in the world. No. You can rely on all American scales to weigh a pound equally, all yards to be the same length, and all pints to have the same volume. But if you want to verify that your foot is exactly one foot, you may have to go to some central location to compare it to the 'official' foot. The base units of the metric system are defined in terms of natural measurements in such a way that they may be reproduced in any scientific laboratory. The metric system also has logical relationships among the measurements of length, area, volume, and mass. Once the meter is established (using the speed of light), area measurements are simply length x width. Building a 1-meter cube and filling it with water gives the tonne. Dividing the cubic meter by 1000 gives you a liter and dividing the tonne by 1000 gives the kilogram.