Eldest rhymes with:
coldest, mildest, oldest, blondest, buddhist, coldest, encyclopedist, fondest, grandest, hardest, kindest, loudest, methodist, mildest, modest, oddest, oldest, orthopedist, prejudiced, propagandist, proudest, rudest, saddest, sadist, shrewdest, soundest, stupidest, weirdest, widest