In what grade are teachers most important?

It depends at what you want to look at. Human beings go through phases of cognitive development where different types of things must be learned. A high school teacher is not very important if your goal is to teach fine motor skills. Conversely, a kindergarten teacher probably wouldn't advise her students on what careers they should consider. Both of these things are very important roles of teachers of two opposite grades, and I would argue these roles are equally important. I have an undergrad in psychology and I was particularly interested in developmental psychology in which I learned that early childhood education is incredibly important at laying the foundation for future learning. Those initial connections in ones brain and forming those initial schemas is vital to being able to continue your learning. Some developmental psychologists argue that early childhood is the foundation of a building - once you've put down the foundation you can't really change the way the building is going to turn out. Sure you can change the types of windows and the colour of the brick and you may even be able to add a wing or a floor later on, but you're building the Eifel Tower atop the basement of a house, and you're not building a house atop the basement of a shopping mall. If you subscribe to those idea and agree with that analogy then I guess you would feel that early childhood education is the most important and anything after that affects relatively minor changes. But ultimately, I think that teachers at all ages are equally important because as young people pass through these developmental stages they need the appropriate guidance and environments along the entire path, not just parts of it.

All I can do is give my opinion. I can't say it's better than another's, nor do I have "concrete" (or objective) research backing my opinion. I think I was most effected by my college professors. More than at any other level. They are the ones who challenged me to think, which helped me to question and discover who I am, and what my purpose is.