H3O+ is considered the strongest acid in aqueous solutions because it results from the protonation of water molecules when a stronger acid donates protons. This process generates hydronium ions, which dominate in acidic environments. Conversely, OH- is the strongest base present in significant amounts in water, as strong bases extract protons from water, leading to an increase in hydroxide ions. The presence of excess water in these solutions facilitates the formation of H3O+ and OH-, establishing their roles as the strongest acid and base, respectively.