Certainliest

Triall doth certainlieſt the truth bewray.

Source: wiktionary

Without doubt many acute, malign, and confuſed ſickneſſes, would be beſt, eaſilieſt and certainlieſt cured by Plants of that Planet that are antipathetical to the Planet that is found to cauſe the Diſeaſe; […]

Source: wiktionary

Next to the Act of Cogitation and Volition itſelf, and to the moſt certain Objects of Sence, there is nothing in all the World ſo Certain, that is, ſo Evident to the Intellect, as the Being of God: He being that to the Mind which the Sun is to the Eye, certainlieſt known, though little of him be known, and no Creature comprehend him.

Source: wiktionary

Uſe always the float and quill, angle for him [the carp] ſometimes above, and ſometimes below mid-water, as the weather is, tho’ in mid-water he is the certainlieſt taken, eſpecially in a pond, but in rivers he is very ſhy: […]

Source: wiktionary