I had a little ride around Burlington, Vermont, on Sunday. 20+ miles. I went up a few moderate hills (in an area aptly nameed "the hill section"), and found 1st gear to be adequate. It's not a MTB granny gear by any means, but I was going up the hills without requiring an AED and oxygen at the top.

I'm guessing roughly a 10% grade at a minimum, but it's hard to tell from Google's terrain maps. One street is called Cliff Street.

Edit: I found a web site indicating Cliff Street averages 14.6%, with a max of 16%.