Why is guanidine considered a strong base?
Wikipedia notes that if a base (in water solution) has a conjugate acid whose $\mathrm{p}K_\mathrm{a}$ exceeds $13,$ it's a strong base.  The criterion is (apparently) based on complete or at least predominant dissociation up to $0.1$ molar.  Guanidine makes it, ergo a strong base.