By far the easiest music notation to work with is Klavar (full name Klavarskribo). Klavar is easy to learn and easy to play particularly with keyboard instruments such as piano, organ or synthesizer. The system was invented by a Dutchman called Cornelis Pot.
Some time in the early 1930s Pot had a brilliant idea: he imagined black lines extending upwards from the black keys of a piano keyboard. He realized that if he could transfer that image onto paper and write dots on the lines or spaces formed by the pattern he could write music in a simple, practical way: it would show the performer exactly which notes to play. Pot published his ideas in 1931 and called his system of writing music Klavarskribo which is Esperanto for "Keyboard Writing". Here I will simply refer to it as 'Klavar'.