Which class when played with skill affects your group the most?

[damstraight4]

I'm curious which class(es) do you think affects your group the most when the player puts in a lot of effort, or is skilled.

For example, you can tell the difference between a great puller and a shit one. A skilled monk will keep your group alive.

Also, a great enchanter keeps the exp flowing if the pulls are up to speed.

What do you guys think?