I'm not an expert, but it has to be Mozart. He really got the ball rolling vis a vis composing operas that were not just mere entertainment. Wasn't Don Giovanni the first piece of music ever to express the emotion of fear unequivocally? I heard someone say that once in a documentary, and it makes sense to me. The climactic scene when the Don is damned to hell not only has fear, the darkness is palpably and vividly portrayed, in both the vocals and the music. Mozart was really one of the first big innovators in the world of opera. The only other one earlier than him that I think of as equally big in terms especially of innovation and imaginativeness was Monteverdi. & not only were Mozart's dramatic operas so advanced, so were his comic/light operas. Of the three in Couchie's poll, I would argue that Wolfgang Amadeus had the broadest range in the operatic realm. That's not denying the impacts that Verdi's and Wagner's masterpieces made, but I'm sure those two giants would have at least tipped their hats to the Austrian any day, and probably would have at least admired & found not a single fault in the score of Don Giovanni...