。
Nothing is greater than a mother's love.
Literal
Mother [gen-の] love [as-much-as-ほど] great thing [topic-は] doesn't-exist.
The construction Xほど~ないものはない is a canonical superlative via double-negative: 'there is no thing that is great to the extent of X' = 'nothing is as great as X.' Extremely common as a structure for making strong comparative claims, and a good example of how Japanese often expresses superlatives by denying alternatives rather than using a dedicated 'most' form.