Spanish as a pure concept might be so, but there are a lot regional variations. 'll' (double l) is /j/ (English y) in most places, but English 'sh' in Argentina - "Sho, mi shamo Raymott". Maybe one of our Spanish speakers can think of other examples where less pure Spanish doesn't quite live up to these ideals. But I do agree that Spanish is the closest language I know of that almost achieves this ideal.
@ Nickelxoxo: To say that Mexico is pronounced Méjico or Mehico really depends on whose /j/ and /h/ you're using. (Apparently, you mean a Spanish pronunciation - Neither works in IPA for English or Spanish.) So, to tell an English audience that a Spanish 'x' is pronounced like a Spanish 'h' or 'j' here might not be that useful.
But for Argentinians, that sound is what the digraph formerly known as 'll' means to them, and it always does, so I would argue that each region still enjoys a one to one relationship between spelling and pronunciation.