。
A mother's love is the greatest.
Literal
Mother [gen-の] love [topic-は] most great is.
もっとも ('most') is one of Japanese's few direct superlative markers — more common in everyday Japanese is the workaround 一番 ('number one'), or comparative-with-denial patterns like Xほど~はない ('there is nothing as ~ as X'). もっとも has a slightly more written feel than 一番, and combined with である it keeps the elevated register.