Softest rhymes with:
swiftest, advantest, anabaptist, anesthetist, apprenticed, artist, baptist, bluntest, brightest, cornettist, cultist, dentist, faintest, fastest, fattest, fittest, greatest, hottest, lastest, latest, librettist, lightest, neatest, noticed, orthodontist, practiced, quietest, quietist, remotest, slightest, smartest, strictest, sweetest, swiftest, tightest, wettest
Softest sounds like:
sabatista, sabotaged, speedskate, subtext, swiftest