Why Guanadine is a strongest base among neutral compounds?

Basicity of Guanidine:

Guanidine is the strongest base among neutral compounds:

The remarkable basicity of guanidine is attributed to the fact that the positive charge on

the guanidinium ion is delocalized equally over the three nitrogen atoms, as shown by

these three equivalent resonating  structures:

Basicity of nitrogen can be increased by attachment to pi-donors (NH₂) group. These two pi-donating NH₂ groups donate electron density to the (pi-accepting) C=NH. Hence, the guanidinium ion is a highly stable cation. 

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