A coworker of mine insisted we stop by Garrett's while we were walking down Magnificent Mile... she said it's TO DIE FOR.
It was a Saturday, so the line was about a 35 minute wait, but man, when I got that bag of caramel popcorn - it made it all worth it.
They make all the popcorn fresh, and I swear they melt straight caramel on the stuff. It was so sticky sweet, it made my teeth stick together. They also have cheese, butter, and plain popcorn too.
They make gift tins that you can take home, and you can also order online for shipping.
One suggestion - if you are going, try to hit it up on a weeknight. They are open until 9:30, and they aren't too busy then. Weekends are an outrageous wait.