In normal circumstances, all can claim to be the best. For, in such conditions there is nothing as a true criterion for investigating.
Like all life states, the same thing is going on in the relationship between Lord and His servants. As you see, there are many people who claim they submit to Allah (SWT) and call themselves Muslims, but Are they really sincere in their claims? Although Allah (SWT) is omniscient and knows everything but it must become clear for servants as well.
For, God legislated a religious order to ascertain people sincerity and fasting in holy month of Ramada