Not a hill I can't climb, but my favorite challenge is Cheyenne Canyon Road in Colorado springs/cheyenne mountain Colorado. Very tough climb to ride well in my opinion. Currently my best time is 20 minutes, lance does it in around 14. Hopin' by end of summer i'll be in the 18's. By the way it is a 3.2 mile climb. I think i've read like 7%ish grade average, but the fact that it starts over 6000 feet and ends over 7000 feet makes it pretty tough.