The Voices will test your watching ability, and certainly your rating criteria. The question to ask is perhaps how well a film manages to become exactly what it was setting out to be. If that's where you're coming from, I find it hard to argue that it isn't perfect.
La La Land isn't just an instant classic, it's an instant litmus test, and if you don't love it, you're going to want to be careful who you tell. There's more movie happening here than you find in the average ten films, and Stone and Gosling can't receive too much praise.
You couldn't shoot out of the gates any better, and The Longest Week is going to make life a lot easier for writer/director Peter Glanz, but that may be a "perfect world" statement. It's a film that is overflowing with talent and style, but it is equally lacking in that which draws large crowds to theaters.