#Hawaiʻian.
The Hawaiʻian language is known for its very small consonant inventory: there are only eight generally-accepted consonant phonemes, each with its own letter in the orthography (P, K, H, M, N, L, W, and ʻ).
The ten* vowels are also represented unambiguously in the orthography. Each short vowel (A, E, I, O, U) is given its own letter, while the long vowels are marked with macra (Ā, Ē, Ī, Ō, Ū).
Thus, each grapheme designates exactly one phoneme, and each phoneme is designated by exactly one grapheme.
On the flip-side, this doesn't mean it's always clear how a word should be pronounced. One side effect of the small consonant inventory is a LOT of free variation; free allophones of /k/ include [t], [s], [d], [z], [ts], [c], [tʃ], [ʃ], [g], and [x], for instance. So when place names and loanwords are borrowed into other languages, they tend to be written more phonetically than phonemically.
* (This number is not entirely agreed-upon. It comes down to whether or not diphthongs and geminated vowels are phonemes. Since diphthongs are still formed productively, some linguists consider there to be only five vowels in the underlying representation. Others count the long vowels as separate, giving ten. A third camp categorizes each diphthong as its own phoneme, for a total of 25. Luckily, these different interpretations agree that the orthography is phonemic; they disagree on whether or not digraphs are necessary.)