I always thought the tallest mountain in Tsuyama was Mount Takiyama.
Literal
Tsuyama most-high mountain [topic-は] Takiyama [copula-だと] thinking was.
津山 (Tsuyama) is a city in Okayama Prefecture. 一番高い山 ('the tallest mountain') uses 一番 for superlative. ~だと思っていました ('I thought [mistakenly] that') uses the past progressive to indicate a belief now corrected. The sentence implies the speaker has since learned differently — a common 'I used to think' structure.