、。
It's usually the islanders who can see the mainland most clearly.
Literal
Mainland [possessive-の] figure [subject-が] most clearly-[adverbial-と] is-visible [nominalizer-の]-[topic-は] usually, island [possessive-の] residents [is-なのである].
A literary-register cleft construction. The full embedded clause 本土の姿が最もはっきりと見える is nominalized with の, then made the topic with は — 'the one who can most clearly see the mainland's figure, (that one) is...'. である is the formal written copula; なのである adds emphatic explanation ('it is the case that...'). The observation is proverb-like: those closest to something may not see it most clearly — sometimes an outside perspective reveals what proximity obscures.